Connect networks to enable multi-mode transfers.

Removing the per-transit type supply routes allows us to find the best
route from A to B even if the unit needs to switch transit modes along
the way.

The "best" route is the one that will generate better gameplay. That is,
convoys are preferred to ships (use cases for GMT are rare in DCS), and
ships are preferred to airlift (reasons to attack cargo ships are also
rare). Avoiding airlift is also a good strategic choice generally since
it consumes aircraft that could be performing other missions.

The extreme weight against airlift in the pathfinding algorithm could
probably be scaled way down so that airlift would be given preference
over a very long trip, possibly only for urgent transfers.

Later when we add rail that will probably be given the most preference,
but possibly between road and shipping.

https://github.com/Khopa/dcs_liberation/issues/823

game/theater/controlpoint.py

@@ -466,6 +466,9 @@ class ControlPoint(MissionTarget, ABC):
     def is_friendly(self, to_player: bool) -> bool:
         return self.captured == to_player
 
+    def is_friendly_to(self, control_point: ControlPoint) -> bool:
+        return control_point.is_friendly(self.captured)
+
     # TODO: Should be Airbase specific.
     def clear_base_defenses(self) -> None:
         for base_defense in self.base_defenses: