Hack P_SetupLevel into little pie- actually just clean it up a bit

I'm currently messing with the level loading code, so I took the opportunity to clean up the mess that is P_SetupLevel a bit. I renamed it to P_LoadLevel because that's a little more accurate to what it does. Also renamed a few other functions called by it, separated out some stuff into subfunctions, simplified some convoluted and/or verbose code etc. A lot more cleanup is needed, but this is what I could do without diving in too deep.

srb2win-levelload-refactor.exe