Reorder map loading

Two changes to the order of map loading that will become necessary with UDMF:

• Move P_ProcessLinedefsAfterSidedefs into P_LoadMapData, where it really belongs
• Move the copying of map data for netplay synchronization closer to the end of P_LoadMapFromFile, after all the functions that mess with the map data have been called.

srb2win-mapdatacopy.exe