Delay vehicle cache init to after map upgrades in load

Split AfterLoadVehicles into two functions.
Vehicle cache init and other functionality requiring an upgraded and
valid map is now performed later in the load process.

Re-order load update for SLV_139, it is no longer required to be
performed before the first phase of vehicle updates

src/elrail_func.h

@@ -37,5 +37,6 @@ void DrawRailCatenaryOnTunnel(const TileInfo *ti);
 void DrawRailCatenaryOnBridge(const TileInfo *ti);
 
 void SettingsDisableElrail(int32_t new_value); ///< _settings_game.disable_elrail callback
+void UpdateDisableElrailSettingState(bool disable, bool update_vehicles);
 
 #endif /* ELRAIL_FUNC_H */