Add new signal type: no-entry signal

This is only passable in one direction, but does not have a signal
or show an aspect in the opposite direction

Add a setting for whether this is shown in the signal UI.
Off by default.

src/rail.h

@@ -47,6 +47,7 @@ enum RailTypeCtrlFlags {
 	RTCF_RESTRICTEDSIG          = 1,                          ///< Custom signal sprite flag enabled for restricted signals.
 	RTCF_NOREALISTICBRAKING     = 2,                          ///< Realistic braking disabled for this track type
 	RTCF_RECOLOUR_ENABLED       = 3,                          ///< Recolour sprites enabled
+	RTCF_NOENTRYSIG             = 4,                          ///< Custom signal sprites enabled for no-entry signals.
 };
 
 struct SpriteGroup;