login: add multi-session X wrapper

In preparation for https://bugzilla.gnome.org/show_bug.cgi?id=655380 we
decided it's better to include the multi-seat X wrapper in systemd,
rather than gdm. (Side effect: this makes this accessible for other
DMs)

This is a stop-gap for now, until X gins proper multi-seat graphics
support at which point this code will go away without replacement.