Consider occupiers for continent threat cache

See bug #22225

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25226 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Cache AI cargo considerations on ruleset load

* Identify ferries, missile carriers, and carriers of occupiers

See patch #4831

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25227 a0f10bec-cc02-0410-94fc-a9cfff90b4cd