Add options to change mode, user, and group of API server socket

[synthead]

Closes #604!

This PR adds three new options to klippy.py, as seen from this truncated help page:

  --api-server-user=APISERVER_USER
                        api server unix domain socket user
  --api-server-group=APISERVER_GROUP
                        api server unix domain socket group
  --api-server-file-mode=APISERVER_FILE_MODE
                        api server unix domain socket file mode

These options allow user to change the user, group, and file mode of the API server socket, i.e. /run/kalico/ud_sock.

For example, these options...

/usr/bin/python /usr/lib/kalico/klippy/klippy.py \
  /etc/kalico/kalico.cfg \
  --input-tty=/run/kalico/sock \
  --api-server=/run/kalico/ud_sock \
  --api-server-group=tty \
  --api-server-file-mode=660

...will create /run/kalico/ud_sock like so:

srw-rw---- 1 kalico tty     0 Mar  5 21:30 ud_sock

From a UNIX permissions perspective, it's a good idea to have Moonraker and Kalico run as different users. Kalico typically has access to hardware, video, network, etc., and if Moonraker runs as the same user, then Moonraker also has access to these things. Moonraker is an API server that should only be communicating with Kalico, and should not have access to the extra privileges Kalico requires. For example, if an RCE in Moonraker is discovered, then the attacker will have access to these elevated permissions.

With this PR, the principle of least permissions can be applied between Kalico, Moonraker, and other software. The socket can be given 660 permissions with a special group, and as long as the Moonraker user is in this group, then it can read/write to the socket without additional permissions.

If the end user doesn't need these features, then no biggie: this PR is not a breaking change, and there will be no behavioral difference before and after this PR without these options set 👍

Checklist

• pr title makes sense
• added a test case if possible
• if new feature, added to the readme
• ci is happy and green

[rogerlz]

nice one. Thanks!