Another OOM killer rewrite: GUI pop-up

Use memory cgroups and their new OOM-kill handler/notifier and put the GUI, X and anything else the GUI program uses to a higher priority cgroup and anything you might want to kill to another cgroup.

Note that normal users don't know what all the (important!) background daemons are so the GUI should probably list for killing only the GUI processes which user has himself opened and which importance he knows.