ukify/man: Look for a config file in systemd folders if not specified

ukify/man: Look for a config file in systemd folders if not specified

If the user does not specify a config file to use, ukify will try looking for one at {/run,/etc,/usr/local/lib,/usr/lib}/systemd/ukify.conf in order and then use the first one found. Also made sure the --config input is a pathlib.Path by specifying its type in its CONFIG_ITEMS entry.
Big cheers to Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl> for helping!