Cancelled member status should be protected from deletion

See http://forum.civicrm.org/index.php/topic,34300.0.html#msg145518

If the cancelled member status is deleted, it breaks new membership signups (at least for anonymous users, any maybe all).

Easiest fix is to simply protect cancelled from deletion (the types pending and deceased are already protected from deletion).

Longer fix would be to modify the membership signup code to not break if it is deleted.