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/newgrf.h

@@ -116,6 +116,7 @@ enum Action0RemapPropertyIds {
 	A0RPI_BRIDGE_PILLAR_FLAGS,
 	A0RPI_BRIDGE_AVAILABILITY_FLAGS,
 	A0RPI_RAILTYPE_ENABLE_PROGRAMMABLE_SIGNALS,
+	A0RPI_RAILTYPE_ENABLE_NO_ENTRY_SIGNALS,
 	A0RPI_RAILTYPE_ENABLE_RESTRICTED_SIGNALS,
 	A0RPI_RAILTYPE_DISABLE_REALISTIC_BRAKING,
 	A0RPI_RAILTYPE_ENABLE_SIGNAL_RECOLOUR,
@@ -123,6 +124,7 @@ enum Action0RemapPropertyIds {
 	A0RPI_ROADTYPE_EXTRA_FLAGS,
 	A0RPI_GLOBALVAR_EXTRA_STATION_NAMES,
 	A0RPI_SIGNALS_ENABLE_PROGRAMMABLE_SIGNALS,
+	A0RPI_SIGNALS_ENABLE_NO_ENTRY_SIGNALS,
 	A0RPI_SIGNALS_ENABLE_RESTRICTED_SIGNALS,
 	A0RPI_SIGNALS_ENABLE_SIGNAL_RECOLOUR,
 	A0RPI_SIGNALS_EXTRA_ASPECTS,
@@ -227,6 +229,7 @@ enum NewSignalCtrlFlags {
 	NSCF_PROGSIG                = 1,                          ///< Custom signal sprites enabled for programmable pre-signals.
 	NSCF_RESTRICTEDSIG          = 2,                          ///< Custom signal sprite flag enabled for restricted signals.
 	NSCF_RECOLOUR_ENABLED       = 3,                          ///< Recolour sprites enabled
+	NSCF_NOENTRYSIG             = 4,                          ///< Custom signal sprites enabled for no-entry signals.
 };
 
 enum {