By default systemd will kill the entire control-group in one fell
swoop. The problem is, things don't get shutdown in the right
order then, and there's a race where GDM will begin restarting
X servers as soon as they're killed.

This commit tells systemd to just kill the main GDM process, and
let it handle killing all its children on its own.

https://bugzilla.gnome.org/show_bug.cgi?id=747088