Kicking a container that had gold in it took the gold amount
away from hero's credit or added to hero's debt, then didn't
give a refund if the container and its gold landed within the
shop. Throwing behaved likewise, just less verbosely.
The problem is caused by addtobill() treating gold specially
and then subfrombill() not being able to perform a reverse
operation. Actually, it may be possible for subfrombill() to
do that, but verifying all its uses is too much work. This
moves the gold handling for drop+selling into its own routine
and adds calls to that for the throwing and kicking refunds.
The other calls to subfrombill() outside of shk.c appear to be
ok as-is. (The calls inside that file are the ones that still
need evaluation if the gold handling is to move to there.)
bill_dummy_object() now uses the same o_id assignment for its
dummy object as split_object() does for its new partial stack.
I don't know whether the old code led to any price glitches.
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