Change encoding of `not in any group' from space to 0, because that's

Change encoding of `not in any group' from space to 0, because that's
friendlier in conditionals:
(army, build_ship, build_land, build_plane, doship, doplane, dounit)
(flee, check_trade, wing, snxtitem_group, takeover_ship)
(takeover_land): Assign 0 instead of space.
(carg, lcarg, pr_ship, pr_plane, pr_land, ldump, land, pdump, plan)
(retr, lretr, sdump, shi, lnd_list, shp_list, ask_olist, att_prompt)
(ask_move_in): Update printing of group.

(takeover_plane): Failed to clear wing.