#164 Unload ally/enemy planes from ships

[Jeff Cotter]

Allies' planes cannot be unloaded from owners ships by
either the owner of the ship or the plane.

If no petrol on ship, eg. when ship can't carry petrol, the
plane becomes stuck.

Ship owners should be allowed to unload whatever is on
their ship, regardless of relations (even AT WAR) if the
usual conditions are met (eg. owns harbor, eff>2%,
etc.).

I looked at load() and load_land_ship() but the
conditional test that prohibits unloading wasn't obvious
to me.