- when a shopkeeper leaves the shop to chase the player, and the player
enters the shop, bill_p is set to an unusual value. bill_p needs to be set
back to a valid value if the shopkeeper re-enters the shop.
- Also, the u.ushops state needs to be updated when a shop becomes tended
again if the player is in the shop.
- introduce a new after_shk_move function to handle this
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