Add the new test TransportDependent to the UnitState requirement type

TransportDependent checks if the unit can't exist outside a transport on the
tile where it currently is located.

Incorporates feedback from Emmet Hikory <persia>

See patch #4894

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25746 a0f10bec-cc02-0410-94fc-a9cfff90b4cd