nixos/lightdm: improve systemd service

These improvements come from shopping around
at what other downstreams have done with their
systemd units and recent changes like [0] to gdm.

Note there's no requries or after on dbus.socket because
settings BusName will set this up automaticallly and
give it a type of dbus.

[0]: https://gitlab.gnome.org/GNOME/gdm/commit/2d57f45962dca4975424c20312d7e04f6ff5d55e