drain_item() always assumed player was responsible, so called
costly_alteration() to adjust shop price of disenchanted item.
If it was unpaid and the effect was caused by a disenchanter
attack rather than by the hero, the feedback was nonsensical.
This also lets a disenchanter hit worn rings, amulet, or blindfold
if no armor gets targetted. Amulets, blindfolds, and most rings
have no charge to be drained, but several types of rings do.
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