Utilities for iterating unit recursive transports and unit recursive cargo.

See gna patch #4973

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25715 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Take in account recursive transport restriction rules in path-finding.

See gna bug #22317

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25716 a0f10bec-cc02-0410-94fc-a9cfff90b4cd