fix #H2504 - dropping shop goods inside engulfer

fix #H2504 - dropping shop goods inside engulfer

This one is only seven years old.  Dropping an unpaid item inside an
engulfer leaves it unpaid and still on bill.  If engulfer is killed,
it ends up unpaid when back on the shop's floor.

Treat dropping an unpaid item into engulfer's inventory as stealing
that item.  You have to pay for it to leave the shop, and like any
other dying monster's inventory, the shopkeeper will take ownership
if it lands on the shop floor when the engulfer is killed.

The 'theft' doesn't anger the shopkeeper and the cost shows up on 'Ix'
as part of "usage fees/other charges" rather than as an itemized used
up item.