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.

src/lib/commands/wing.c

@@ -60,7 +60,7 @@ wing(void)
 	return RET_SYN;
     }
     if (c == '~')
-	c = ' ';
+	c = 0;
     if (!snxtitem(&nstr, EF_PLANE, player->argp[2]))
 	return RET_SYN;
     count = 0;