crash fix: shopkeeper re-entering shop

- 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

doc/fixes34.1

@@ -72,6 +72,7 @@ travel command should restrict its shortest paths to areas of the map the
 non-altar prayer should limit god action, not maximize it
 potions of acid explode, not dilute, so make water_damage behave this way
 lookat monster notes if you see monster is trapped
+don't crash when angry shopkeeper re-enters the shop and you pick up something
 
 
 Platform- and/or Interface-Specific Fixes