libpq work

Well after hacking around on libpq for a few days I'm starting to
understand some of the subtley of what's going on. I'm also
understanding that it's complex enough that I'm not going to be sure
anymore about what subtleties I might break. It's a pity there isn't
regression tests for this so I know for sure what is expected to happen,
and what subtle features are in there.

Anyway for this reason, I think I'm going to go back and simply add the
features I want to libpq. I've learnt enough that I think I know now how
to do it, with a minimum of patches.

Longer term, I'd like to implement SQL3 on top of libpq, then get some
comprehensive test harnesses in place, and then slowly dissolve libpq.
But without those regression tests, it's a bit scary. I assume that the
psql tests wouldn't fully exercise libpq what with the asynchonous stuff
etc?