(upd_ship, upd_land, do_plague): Work directly on item arrays instead

(upd_ship, upd_land, do_plague): Work directly on item arrays instead
of copies made by getvec().  This is safe, because the new code
changes the item array precisely when the old code writes back a
changed copy.
(starv_ships, starv_units): Replace getvec() by direct, read-only item
access.
(feed_ship, feed_land): Remove parameter vec, work with sp->shp_item[]
and lp->lnd_item[] instead.
(plague_people): Change argument type to match uncopied item arrays.