Consolidate AI ferry tests

Consider a unit a ferry if it can move in the ocean, and carry another
unit that is not a missile and either cannot move freely in the ocean
or requires fuel.

See patch #4681

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25192 a0f10bec-cc02-0410-94fc-a9cfff90b4cd