[Pgbuildfarm-members] patches for pgbuildfarm client code and docs

I got the pgbuildfarm client code working the other day; there were a
few rough spots where I had to poke through the code to figure out
what was going wrong, and I thought I'd share some suggestions and
fixes based on my experience.

Here are some issues I've attempted to fix:

* For some reason, the --with-ldap, --with-libxml, and
--with-libxslt get forcibly added in build-farm.conf as configure
options even when config_opts claims to have them turned off. This led
to head-scratching configure errors for me ("error: 'choke'
undeclared") until I noticed the extra flags to ./configure in
configure.log.

* I've tried to clear up the PGBuildFarm-HOWTO.txt in a few places.
I think that it would be helpful to include this file inside the
release tarballs.

* The default CCACHE_DIR points to /home/andrew/... I think this
would be more helpful if initialized to undef, disabling the use of
ccache unless the user fills in this config.

I've attached two patches: one with clean-up for the existing
client-code from CVS, and a separate diff against the existing
PGBuildFarm-HOWTO.txt.

HTH,
Josh