Allow cupsd daemon to exit on idle

Allow cupsd daemon to exit on idle

When using socket activation cups daemon can safely exit on idle and be automatically started when it's needed. Using Restart=always prevent this behavior and causes cups daemon constantly restarting itself instead of exiting. Users reported[1] that Restart=on-failure was enough to fix issues[2] so Restart=always may be unnecessary and harmful.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=861470
[2] https://github.com/apple/cups/issues/5263