Add the MF_REVIVES flag

As the title text, this adds a MF_REVIVES flag which can be added/removed from monsters.
Also edits the zombie necromancer mattack, they will now only revive things that can both revive normally and are zombies (which at the moment is no different from previous behavior, but if a new monster was modded in there would be no guarantee that necromancers would revive them).