restructure libpq docs (WIP)

I'd like to improve the libpq documentation, as it is (IMHO) not very
good at the moment.

The first thing I'm working on is restructuring the libpq docs to use
the DocBook "refsect" tags -- so that each function is documented in a
page similar to the existing reference documentation for SQL commands,
with separate sections for the function prototype, parameters, return
value, and description. I've done this for most (80%+) of the functions
in the libpq docs, but a few sections still need to be converted
(asynch. query processing and COPY-related functions).

While doing this, I've also made some miscellaneous improvements to the
libpq documentation throughout, improved some of the markup in
ecpg.sgml, arch-dev.sgml, and history.sgml, and made some
cosmetic-but-pervasive changes to the SGML in libpgctl.sgml.

I figured I'd send this in now, so that people can take a look at the
resulting documentation layout -- if you'd like me to do things
differently or if you have a suggestion, please speak up.

The patch is attached -- it's 420KB uncompressed, so I've gzip'ed it.

Cheers,

Neil
--
Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC