Test nativity of moves

    While the nativity of the tiles for a move is tested, the nativity
of the actual move is not tested.  As a result, in the event that nativity
is provided by a road, the move_mode is not enforced for the provision of
nativity, possibly causing odd results, such as unit classes native to
rivers being able to cross continents.

Requested by Jacob Nevins <jtn>
Based on earlier work by Marko Lindqvist <cazfi>

See bug #16383

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24826 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Use a unit class flag for airlifting

    Rather than restricting airlifts to ground units, provide a
unit class flag that allows ruleset authors to control which unit
classes are considered suitable for airlift.

Implementation suggestion by David Fernandez <bardo>

See patch #3871

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24827 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Permit Barbarian leaders to have amphibious cohorts

See patch #4651

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24828 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Avoid PF_IMPOSSIBLE_MC targets in find_beachhead.

Also avoid duplicate tile analysis

See bug #21913

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24829 a0f10bec-cc02-0410-94fc-a9cfff90b4cd