Put monsters into limbo if they cannot be placed

Put monsters into limbo if they cannot be placed

Whenever mnearto tries to displace a monster from underneath
another, and the displaced one cannot be placed anywhere,
make it drop special objects and put it into migration, with
the current level as the target.

This should be a good enough stopgap measure - it's not going
to happen unless the level is (nearly) full of monsters.

And it seems to cure a near-impossible-to-track data corruption,
with monster list pointing to garbage.