Move stuff from ipx.h to print-ipx.c; it's not used elsewhere.
Move stuff from tftp.h to print-tftp.c; it's not used elsewhere.
Commit more changes from the previous two.
Pull a bunch of headers into the only source file that includes them.

For headers included in only one source file, put the header contents in
the source file in question, and get rid of a bunch of stuff from the
header not used in the source file.
Move the contents of arcnet.h to print-arcnet.c.

It's not used by any other file.
Move stuff from igrp.h to print-igrp.c.
Use struct toks for various enumerated fields.
refresh INSTALL.txt

Don't refer to the source code of GCC, which comes packaged in most
cases today. Delist .h files that don't exist.