Fear transporters of occupiers

    Consider any transporter capable of transporting units that
can take over cities as a potentially threatening transporter,
rather than just transporters of ground units.

See bug #21892

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24776 a0f10bec-cc02-0410-94fc-a9cfff90b4cd