doc: prefer fop to generate pdf from docbook

doc: prefer fop to generate pdf from docbook

It would seem xsltproc -> fop -> pdf is the "modern" way to generate
pdf from docbook now.  The hard part is finding the stylesheets.
This should work for fedora, sles and debian.  Additionally, it brings
some consistency--xsltproc for all the conversions.  You can still
override via configure options if you prefer something else.

Change-Id: Id779e9473a6759daddc9a61be714109b27da980e
Reviewed-on: http://gerrit.openafs.org/4821
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>