Allow articulated road vehicles to overtake other vehicles

Add setting

src/saveload/extended_ver_sl.h

@@ -70,6 +70,7 @@ enum SlXvFeatureIndex {
 	XSLFI_DUAL_RAIL_TYPES,                        ///< Two rail-types per tile
 	XSLFI_CONSIST_SPEED_RD_FLAG,                  ///< Consist speed reduction flag
 	XSLFI_SAVEGAME_UNIQUE_ID,                     ///< Savegame unique ID
+	XSLFI_RV_OVERTAKING,                          ///< Roadvehicle overtaking
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk