systemd is a tricky beast: getting it to actually launch gdm
required considerable effort on the part of stupid myself.

The relevant links:
* https://wiki.archlinux.org/index.php/Systemd#Running_DEs_under_systemd
* http://unix.stackexchange.com/a/35626/15296

Still looks pretty dirty, and there's some duplication
with a hook in features.in/slinux as well.