When building, install dependencies' docs

Mainly because hackage doesn't, at the moment.
Put MD5Auth in its own module, reexported from Utils
More pgpass stuff

The upshot of which is that Utils.lookupPassword should be the one-stop
method for getting a password from a .pgpass file.

Finding it is not quite correct -- it trusts $HOME on unix, and does
not look in the right place on Windows.

That place-finding should be pulled out into its own thing if we want
to support a PGSERVICEFILE-like thing too.
OS-specific config dir lookup -- windows is completely untried

..which means there is at almost certainly some stupid error in
there somewhere.
Rearrange ifdefs again
Hey there's a "win32" package that already binds SHGetFolderPath

Still untested, so probably there's some typo in there.