This was to resolve some circular dependencies with the portal code.
The most notable changees:
* FTextureID was moved from textures.h to doomtype.h because it is frequently needed in files that don't want to do anything with actual textures.
* split off the parts from p_maputl into a separate header.
* consolidated all blockmap related data into p_blockmap.h
* split off the polyobject parts into po_man.h
