pid1: make MaxConnectionsPerSource= also work for AF_UNIX sockets

pid1: make MaxConnectionsPerSource= also work for AF_UNIX sockets

The setting currently puts limits on connections per IP address and
AF_UNIX CID. Let's extend it to cover AF_UNIX too, where it puts a limit
on connections per UID.

This is particularly useful for the various Accept=yes Varlink services
we now have, as it means, the number of per-user instance services
cannot grow without bounds.