Updates for docbook builds on Windows

In config.nmake search for multiple versions of fop in a couple of places, preferring fop 1.1
In Makefile.nmake remove any PYTHONPATH env var (as that's probably for Windows)
and not the Cygwin python used in asciidoc
In Readme.txt update some Cygwin package info

Change-Id: I117ec6bbdabc34829b421fd514508383b925090f
Reviewed-on: https://code.wireshark.org/review/3816
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>