Fix reading bug--wasn't reinforcing morale bonuses if the expected int was
above current int.
Migrate player_activity from invlet to position.  This would break
compatibility with saves that had an ongoing activity with an associated
invlet, but there aren't any usecases I know of that allow it (since
activities take control focus, and saving requires focus).
Migrate first aid to using position directly, add player::reduce_charges
to a fix pre-existing bug where first aid wouldn't be consumed when wielded.