Add on-leave toggle for pilots.

Pilots on leave will not be assignable to any flights (but will not be
unassigned from any already scheduled this turn).

https://github.com/dcs-liberation/dcs_liberation/issues/276

game/event/event.py

@@ -126,7 +126,7 @@ class Event:
                 not loss.pilot.player
                 or not self.game.settings.invulnerable_player_pilots
             ):
-                loss.pilot.alive = False
+                loss.pilot.kill()
             aircraft = loss.flight.unit_type
             cp = loss.flight.departure
             available = cp.base.total_units_of_type(aircraft)