server: invoke X with the systemd multi seat X wrapper if necessary

systemd 39 and newer provide a small wrapper for X which works around
the fact that XOrg upstream currently support multi-seat hotplug for
displays. Let's make use of this as a stop-gap until this feature is
added to XOrg upstream.

This code tries to be as defensive as possible and makes use of the
wrapper only if the system as actually booted with systemd, the wrapper
is available and we actually use a seat != "seat0".