Fix: bill_dummy_obj billed excessively for stacks

Add a way to request that unpaid_cost() produce the cost for a single
item, which is necessary for the price adjustment made in
bill_dummy_object.  Another option would be to simply divide by quan in
bill_dummy_object, but this might be more future-proof in case
unpaid_cost ever involves more than simple multiplication by quan
(e.g. the use of alternate units vs the base price, as are used for
globs).

Fixes #1236

src/mkobj.c

@@ -710,7 +710,7 @@ bill_dummy_object(struct obj *otmp)
     long cost = 0L;
 
     if (otmp->unpaid) {
-        cost = unpaid_cost(otmp, FALSE);
+        cost = unpaid_cost(otmp, COST_SINGLEOBJ);
         subfrombill(otmp, shop_keeper(*u.ushops));
     }
     dummy = newobj();