Add reserve through action to trace restrict programs.

This only changes the behaviour of PBS reservations which
would otherwise terminate at a PBS signal.
If the signal restriction sets the reserve through state,
the reservation continues through the signal, and the signal
is set to green.

src/pbs.h

@@ -22,6 +22,7 @@ TrackBits GetReservedTrackbits(TileIndex t);
 void SetRailStationPlatformReservation(TileIndex start, DiagDirection dir, bool b);
 
 bool TryReserveRailTrack(TileIndex tile, Track t, bool trigger_stations = true);
+bool TryReserveRailTrackdir(TileIndex tile, Trackdir td, bool trigger_stations = true);
 void UnreserveRailTrack(TileIndex tile, Track t);
 
 /** This struct contains information about the end of a reserved path. */