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

include/hack.h

@@ -308,6 +308,13 @@ enum cost_alteration_types {
     COST_CRACK   = 19, /* damage to crystal armor */
 };
 
+/* used by unpaid_cost(shk.h) */
+enum unpaid_cost_flags {
+    COST_NOCONTENTS = 0,
+    COST_CONTENTS   = 1,
+    COST_SINGLEOBJ  = 2,
+};
+
 /* read.c, create_particular() & create_particular_parse() */
 struct _create_particular_data {
     int quan;