textures: Only include patches needed by textures.

Now that #1 is fixed, we can be certain that all patches needed for
compatibility are definitely being included in the PNAMES lists. It
therefore isn't necessary to include every patch in the patches/
directory in every WAD.

Extend the build-textures script to generate a text file containing
the list of PNAMES, and include this from the main config file. That
way, each IWAD only gets the patches it explicitly needs.
doc: Update BUILD-SYSTEM

Many of the details in this file are now obsolete. Update the
documentation to explain how things are now built (now that we have
a custom texture builder) and include details of some of the other
build scripts in subdirectories.