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

include/extern.h

@@ -1761,6 +1761,7 @@ E struct monst *FDECL(shkcatch, (struct obj *,XCHAR_P,XCHAR_P));
 E void FDECL(add_damage, (XCHAR_P,XCHAR_P,long));
 E int FDECL(repair_damage, (struct monst *,struct damage *,BOOLEAN_P));
 E int FDECL(shk_move, (struct monst *));
+E void FDECL(after_shk_move, (struct monst *));
 E boolean FDECL(is_fshk, (struct monst *));
 E void FDECL(shopdig, (int));
 E void FDECL(pay_for_damage, (const char *));