constrain monster migration in wizard tower

constrain monster migration in wizard tower
<Someone> noticed that when a monster escaped upladder in the wizard
tower, it ended up outside the tower.  This is due to the "wander" code in
monster migration.  Rather than add code to try to keep the monster from
crossing the undiggable wall, just add REGIONs on the tower levels within the
area, which then utilizes the existing in-a-room constraint behavior of
monster migration. Of course, one can still fill a tower level with fodder,
and then when another monster climbs the ladder, it will still end up
outside the tower.