Add reverse at waypoint orders.

The train will reverse when its tail is within the waypoint tile.
This is useful for reversing on train en-route, without creating
dedicated reversing sidings or platforms.

src/saveload/extended_ver_sl.cpp

@@ -45,6 +45,7 @@ std::vector<uint32> _sl_xv_discardable_chunk_ids;           ///< list of chunks
 static const uint32 _sl_xv_slxi_chunk_version = 0;          ///< current version os SLXI chunk
 
 const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
+	{ XSLFI_REVERSE_AT_WAYPOINT,    XSCF_NULL,              1,                                    1, "reverse_at_waypoint",   NULL, NULL, NULL        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };