bcache-register: use register, not register_quiet

register_quiet doesn't report errors correctly, the following kernel
patch needs to be applied:  https://lkml.org/lkml/2013/8/26/549

See http://thread.gmane.org/gmane.linux.kernel.bcache.devel/2594
for how this comes up in practice.

Reverts 8327108

Do not compile bcache-test by default

This way the Debian package can build without OpenSSL.

Take user-modified rules from /etc

Copy udev rules from /etc/udev/rules.d/ if they have been user-modified,
otherwise copy from /lib/udev/rules.d/ instead. Follows usual udev
conventions for modifying rules files.

Update the README to mention udev and initramfs hooks

udev: Skip DM devices marked private

Add an INSTALL macro to the Makefile

udev: Fit into the standard rule sequence

Move the rule to run a bit later, after
the standard udev rules have called blkid.

Don't run blkid manually, and run probe-bcache
as a fallback which can be patched out in a
package that depends on util-linux 2.24+.

probe-bcache: bail if libblkid finds anything
Preserve our last-resort safety check here
in case earlier rules fail to call blkid.

Install the initramfs hook in /usr/share not /etc