The gdm user session that runs gnome-session, gnome-shell and plenty of gnome-settings-daemon helpers should be terminated when not in use.

On some low-memory devices, it might not otherwise be possible to log in, as the gdm session takes much of the available RAM. (As seen by Hans on 1GB and some 2GB devices)

Possible heuristics for this change:
- always kill the session, it will make fast-user switching slower when there's loads of memory available
- always kill it when there's only one user account on the machine
- always kill it when there's less than a certain amount of memory (I'd say less than 4GB, but less than 2GB is the bare minimum)

Or combinations of the above.

Note that this wouldn't affect gdm's availability for helping gnome-shell's lock screen to authenticate users.