Reported a while back, a (stonable) hiding monster will hide at a location
containing only a cockatrice corpse. While it would be interesting to
allow monsters to try, and stone themselves as a result, I chose the
simpler fix which is to not have monsters hide in such situations. I found
the hiding code was duplicated in several places, so I moved it into a new
hideunder() function that works for both the hero and monsters.
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