Refactor map loading functions

Based on !580 (merged), so go and review that one first.

Clean up and standardize the map loading functions. I renamed a lot of them to be more descriptive of what they actually do.

The main functional change is that the FOF flags for the custom FOF are now processed immediately in P_LoadSidedefs and stored in the back upper texture field, instead of reloading the sidedef data in P_SpawnSpecials. Aside from just being cleaner and simpler, this also makes the eventual binary-to-UDMF conversion easier. In order to achieve this, sidedefs now store a pointer to the linedef they belong to, so that P_LoadSidedefs can figure out if it's processing a front or back side.

srb2win-mapload-refactor.exe