pull request #888 - latent shop creation bug

Pull request from copperwater:  if a theme room produced a room with
one door that effectively opened on a 'hallway' leading to the core
of that room, it would be considered to be eligible to become a shop.
But if the only spot available for the shopkeeper to move away from
the spot in front of the door was another spot in the hallway, there
would be no possibility to get out of the way, either to let someone
in, or if the hero arrived by teleport or trap, to let hero out.
|     --
|  ---..[rest
|##+12..of the
|  ---..room]
|     --
Shopkeeper would move back and forth between 1 and 2, always blocking
access between the door and the rest of the room.

Currently no rooms get generated with that shape.  I haven't tried to
force one in order to verify the fix.

Fixes #888