Refactor map loading functions

See merge request STJr/SRB2!594

Eliminate all unnecessary uses of ZSHIFT

See merge request STJr/SRB2!580

Store custom FOF flags directly in back toptexture field at map load instead of re-reading the sidedefs data later on

Add missing MFE_ flags to Lua (resolves #103)

See merge request !592

-Add linedef pointer to side_t, so sidedefs are able to tell if they're a front or back sidedef during setup
-Fix a broken condition during setup of texture fields for the change music linedef

This reverts commit 2cf94ea6.

Fix logfiles crashing Windoze (resolves #102)

Closes #102

See merge request !591

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

See merge request STJr/SRB2!587

Remove MF_NOTHINK from join-breaking referenced objects

See merge request STJr/SRB2!589

Rename P_LoadThingsOnly to P_RespawnThings to make it clearer that it doesn't actually reload the things from the file, it just respawns them.

Rename P_SetupLevel to the more accurate P_LoadLevel and separate out some of its content into subfunctions

More rollangle changes

See merge request !582