Pull request #654 from entrez: the change to avoid impossible
"unpaid_cost: object wasn't on any bill" when examining inventory
while in a wall shared by multiple shops didn't work correctly for
shop-owned objects inside hero-owned containers (which worked prior
to that change).
Not fixed, and possibly not going to be: if a hero-owned container
holds items from more than one shop, the cost appended to those items'
inventory entries might show the wrong amount if the relevant code
picks the wrong shk. Presumably the wrong amount can also happen if
player puts items from one shop into a container owned by another
shop.
Closes #654
when two or more shops share a wall and hero uses Passes_walls to carry an
unpaid item through the shared wall into another shop, theft of that
unpaid item from the first shop wasn't noticed
+redo the unpaid_cost fix to handle shop items inside hero-owned container
flyers shouldn't fall on arrival when going down holes or trap doors