Revise approach to NUT config directory options

[jimklimov]

Closes: #3131

Should simplify life for packagers, improve autotools standards compliance, and be invisible to existing users and their build recipes.

• --with-confdir-suffix depends on custom-or-not prefix and sysconfdir, defaults to empty or nut (resolved)
• --with-confdir defaults to ${sysconfdir}/${confdir_suffix}
• --sysconfdir should no longer be tweaked
• --with-confdir-examples allows *.conf.sample files to land elsewhere, if distro policy wants that

[AppVeyorBot]

✅ Build nut 2.8.4.3604-master completed (commit 5b6eab7a08 by @jimklimov)

[jimklimov]

Tests:

• Completely out-of-the-box with no options, works like before:

:; ./configure
...
checking confdir-suffix to use after sysconfdir (${prefix}/etc)... prefix was not customized... ''
...
* Default installation prefix path:     /usr/local/ups
...
* Config file path:     /usr/local/ups/etc
* Config file examples path:    /usr/local/ups/etc
...

:; make DESTDIR=/dev/shm/nut-oob -j 8 install

:; find /dev/shm/nut-oob/ | grep etc
/dev/shm/nut-oob/usr/share/augeas/lenses/dist/nutupssetconf.aug
/dev/shm/nut-oob/usr/local/ups/etc
/dev/shm/nut-oob/usr/local/ups/etc/ups.conf.sample
/dev/shm/nut-oob/usr/local/ups/etc/nut.conf.sample
/dev/shm/nut-oob/usr/local/ups/etc/upsd.users.sample
/dev/shm/nut-oob/usr/local/ups/etc/upsmon.conf.sample
/dev/shm/nut-oob/usr/local/ups/etc/upssched.conf.sample
/dev/shm/nut-oob/usr/local/ups/etc/upsd.conf.sample

:; /dev/shm/nut-oob/usr/local/ups/bin/al175 -DDDDDD -a ttt
   0.000000     [D5] send_to_all: SETINFO driver.state "init.starting"
Network UPS Tools 2.8.4.317.2-319+ga25741011 (development iteration after 2.8.4) - Eltek AL175/COMLI driver 0.18
Warning: This is an experimental driver.
Some features may not function correctly.

   0.000124     [D5] do_upsconf_args: confupsname=dummy, var=driver, val=dummy-ups
   0.000164     [D5] do_upsconf_args: confupsname=dummy, var=port, val=auto
   0.000202     Error: Section ttt not found in ups.conf
   0.000242     [D5] send_to_all: SETINFO driver.state "cleanup.exit"
   0.000276     [D1] upsnotify: notify about state NOTIFY_STATE_STOPPING with libsystemd: was requested, but not running as a service unit now, will not spam more about it
   0.000323     [D1] upsnotify: failed to notify about state NOTIFY_STATE_STOPPING: no notification tech defined, will not spam more about it

### This actually looked into existing /usr/local/ups/etc files (ignored DESTDIR as it should)

• old recipe tweaking sysconfdir (works like before, adds a notice in the end):

:; ./configure ... --sysconfdir=/etc/nut ...
...
checking how to "tail" starting from Nth line... /usr/bin/tail  +NUM_FIRST_LINE
checking confdir-suffix to use after sysconfdir (/etc/nut)... sysconfdir was customized... ''
checking for realpath... realpath
configure: This system seems to prefer '/run' over '/var/run': adjusting default PIDPATH
...
* Config file path:     /etc/nut
* Config file examples path:    /etc/nut
...
* CONFIG_FLAGS  : --with-user=nut --with-group=nut --with-all --with-docs=man --sysconfdir=/etc/nut --enable-silent-rules --enable-Werror
configure: ==========================================================
configure: This build configuration modifies the --sysconfdir setting.
configure: While this was the way for decades, it was the wrong way.
configure: Consider updating your build recipes to use --with-confdir*
configure: options nowadays.
configure: ==========================================================

• Using full confdir spec (and evaluatable verbatim pattern for the examples) INSTEAD of sysconfdir:

:; ./configure --with-confdir=/etc/ups --with-confdir-examples='${docdir}/sample-conf'
...
configure: A non-trivial --with-confdir path was specified (/etc/ups); any --with-confdir-suffix (auto) or --sysconfdir (${prefix}/etc) values are disregarded
...
* Default installation prefix path:     /usr/local/ups
...
* Config file path:     /etc/ups
* Config file examples path:    /usr/local/ups/share/doc/nut/sample-conf
...

:; make DESTDIR=/dev/shm/nnnccc -j 8 install

# Check CONFPATH_EXAMPLES:
:; find /dev/shm/nnnccc | grep samp
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf/upsmon.conf.sample
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf/upssched.conf.sample
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf/ups.conf.sample
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf/nut.conf.sample
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf/upsd.users.sample
/dev/shm/nnnccc/usr/local/ups/share/doc/nut/sample-conf/upsd.conf.sample

# Check built-in CONFPATH:
:; /dev/shm/nnnccc/usr/local/ups/bin/al175 -DDD -a ttt
Network UPS Tools 2.8.4.317.2-319+ga25741011 (development iteration after 2.8.4) - Eltek AL175/COMLI driver 0.18
Warning: This is an experimental driver.
Some features may not function correctly.

   0.000000     Can't open /etc/ups/ups.conf: Can't open /etc/ups/ups.conf: No such file or directory
   0.000083     [D1] upsnotify: notify about state NOTIFY_STATE_STOPPING with libsystemd: was requested, but not running as a service unit now, will not spam more about it
   0.000121     [D1] upsnotify: failed to notify about state NOTIFY_STATE_STOPPING: no notification tech defined, will not spam more about it

• Using just a suffix:

:; ./configure --with-confdir-suffix=ups --with-confdir-examples='${docdir}/sample-conf'
...
checking confdir-suffix to use after sysconfdir (${prefix}/etc)... normalized caller-provided suffix... '/ups'
...
* Default installation prefix path:     /usr/local/ups
...
* Config file path:     /usr/local/ups/etc/ups
* Config file examples path:    /usr/local/ups/share/doc/nut/sample-conf
...

# Check built-in CONFPATH:
:; /dev/shm/nnnccc/usr/local/ups/bin/al175 -a test
Network UPS Tools 2.8.4.317.2-319+ga25741011 (development iteration after 2.8.4) - Eltek AL175/COMLI driver 0.18
Warning: This is an experimental driver.
Some features may not function correctly.

Can't open /usr/local/ups/etc/ups/ups.conf: Can't open /usr/local/ups/etc/ups/ups.conf: No such file or directory

• A prefix explicitly passed on command line (even if same as configure.ac default), sysconfdir untouched: now there is reason to append confdir_suffix (default), and we end up with /usr/local/ups/etc/nut (as expected for this case):

:; ./configure --prefix=/usr/local/ups
...
checking confdir-suffix to use after sysconfdir (${prefix}/etc)... prefix was customized... '/${PACKAGE_NAME}'
...
* Default installation prefix path:     /usr/local/ups
...
* Config file path:     /usr/local/ups/etc/nut
* Config file examples path:    /usr/local/ups/etc/nut
...

:; make DESTDIR=/dev/shm/nut-default -j 8 install

:; find /dev/shm/nut-default/ | grep etc
/dev/shm/nut-default/usr/share/augeas/lenses/dist/nutupssetconf.aug
/dev/shm/nut-default/usr/local/ups/etc
/dev/shm/nut-default/usr/local/ups/etc/nut
/dev/shm/nut-default/usr/local/ups/etc/nut/ups.conf.sample
/dev/shm/nut-default/usr/local/ups/etc/nut/nut.conf.sample
/dev/shm/nut-default/usr/local/ups/etc/nut/upsmon.conf.sample
/dev/shm/nut-default/usr/local/ups/etc/nut/upsd.users.sample
/dev/shm/nut-default/usr/local/ups/etc/nut/upssched.conf.sample
/dev/shm/nut-default/usr/local/ups/etc/nut/upsd.conf.sample

:; /dev/shm/nut-default/usr/local/ups/bin/al175 -a ttt
Network UPS Tools 2.8.4.317.2-319+ga25741011 (development iteration after 2.8.4) - Eltek AL175/COMLI driver 0.18
Warning: This is an experimental driver.
Some features may not function correctly.

Can't open /usr/local/ups/etc/nut/ups.conf: Can't open /usr/local/ups/etc/nut/ups.conf: No such file or directory

• Customized sysconfdir as an evaluatable pattern: currently not different from explicit path there (for legacy recipe compatibility), the resulting path is used directly for NUT configs:

:; ./configure --sysconfdir='${prefix}/custom-etc-configs'
...
* Default installation prefix path:     /usr/local/ups
...
* Config file path:     /usr/local/ups/custom-etc-configs
* Config file examples path:    /usr/local/ups/custom-etc-configs