Make 32 instead of 16 rail types possible. by using one free bit of m1 in the map array.

(cherry picked from commit b44843800880cdc186f5183301ee0f42afbb5f84)

Also set non-rail type M3 bits to 0 where applicable.
Change savegame versioning to SLXI format.

src/saveload/extended_ver_sl.h

@@ -43,6 +43,7 @@ enum SlXvFeatureIndex {
 	XSLFI_VEH_LIFETIME_PROFIT,                    ///< Vehicle lifetime profit patch
 	XSLFI_LINKGRAPH_DAY_SCALE,                    ///< Linkgraph job duration & interval may be in non-scaled days
 	XSLFI_TEMPLATE_REPLACEMENT,                   ///< Template-based train replacement
+	XSLFI_MORE_RAIL_TYPES,                        ///< Increased number of rail types
 
 	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