(carrier_planes): New.

(carrier_planes): New.
(pln_onewaymission): Use it.  No functional change.
(pln_airbase_ok): Use it to enforce capabilities.  Before, a plane
could fly once it got on the ship.  Because ships can load only planes
that can fly from them, this has no effect other than make that sanity
check redundant; remove it.

(pln_airbase_ok): Update land unit case to keep it similar to the ship
case.