systemd v247 now puts services into app.slice by default. But app.slice
will have a conflict with exit.target, which in turn means that we still
get a conflict with exit.target indirectly.

Fix it by adding the appropriate Slice=-.slice into
gnome-session-restart-dbus.service. Also update the note in
gnome-session-shutdown.service to point out the requirements.

Closes: #74