Fix very strange occurances when adjusting items.

When picking up a previously dropped item, correctly update its inventory
slot. Otherwise we end up with an internal inconsistency, where the item's
position in you.inv and the value of item.link doesn't match up with the
value of item.slot, leading to bad behaviour when we try to adjust.
(cherry picked from commit 49c2f50a475cf69030c413fea0dffe76fcdc5a86)
Merge remote-tracking branch 'upstream/stone_soup-0.11' into gettextized-0.11