pam_systemd: Check also abstract socket for X11

pam_systemd: Check also abstract socket for X11

It seems that `pam_systemd` was the only thing left that wanted to use sockets
in file system path `/tmp/.X11-unix/X*`. X11 apps actually prefer using the
abstract socket version.

This allows running Xserver with `-nolisten tcp -nolisten unix`, which makes
the server only listen to an abstract socket.

Also in my setup, Xserver is running as a separate system service instead of
starting from display manager service, and now `PrivateTmp=yes` can be used for
both. The file system of the display manager service is inherited by user apps
and now their `/tmp` will be separate from `/tmp` of PID1 namespace as well as
`/tmp` of Xserver.