(buil): Work directly on item arrays instead of copies made by

(buil): Work directly on item arrays instead of copies made by
getvec().  This is safe, because the old code made single copies and
always flushed them back into the unit structures before flushing
those.
(build_ship, build_land, build_bridge, build_nuke, build_plane,
build_tower): Change parameter type to match uncopied item arrays.