This fixes a bug where the hero could accidentally donate the contents
of their bag to a shopkeeper if they put it in another bag on the shop
floor that also belonged to the hero. To reproduce:
1. Drop a sack on the floor, but don't sell it.
2. Get another sack and put in hero-owned objects.
3. Put the sack with objects into the sack on the shop floor.
4. Take out the sack with the objects from the sack on the shop floor.
The shopkeeper will claim you owe them for the objects in the sack, and
view the contents of the sack will show them as belonging to the
shopkeeper.
This fix is what those previous fixes for `SELL_DONTSELL` were for.
Based on DynaHack commit f91ce0b (Fix billing/credit when hero nests
their containers on a shop floor) by me.
There's no capacity for the shop logic to handle gold without also
changing the credit/debit within it, so gold must always be handled in
`sellobj()`, even when the state of it is set to `SELL_DONTSELL`.
This is needed for an upcoming bug fix.
Based on DynaHack commit b0784c5 (Credit/debit gold in containers even
in sellobj_state SELL_DONTSELL) by me.
For a shop to NOT charge for an object, two conditions apply:
1. The object's `no_charge` flag must be set.
2. That `no_charge` flag must be set regardless of whether or not the
shop typically sells the object in question.
There are two places in `sellobj()` which ignore the second condition,
thus transferring object ownership from the player to the shop without
the player's consent:
1. A container is dropped in a shop that typically sells such
containers and `sellobj_state` is `SELL_DONTSELL`.
2. A zero-cost container holding nothing but gold is dropped in a shop
that typically sells such containers.
Neither occurs currently in NetHack: the latter because NetHack has no
zero-cost containers, but the former is needed for an upcoming bug fix.
This may be related to SC343-21: "Accounting is incorrect for containers
dropped in a shop that does not sell them."
Based on DynaHack commit 4e79b6a (Don't shop-donate non-empty bags
dropped in sellobj_state SELL_DONTSELL) by me.
Tung Nguyen