Updated kerberos service name patch

Here is an updated version of the patch from
http://candle.pha.pa.us/mhonarc/patches2/msg00025.html. It handles the
options for libpq connections the same way other options are handled,
and it also updates the kerberos documentation. It contains a couple of
minor changes to the Kerberos documentation that's not directly related
to this patch, to make it easier to read. And it updates the Kerberos
information URL to the current MIT pages.

I refactored my own code so now the Kerberos 4 specific changes are very
small. I have not verified them, but I think they shuold work. That
doesn't mean I'm still in favour of ripping out the krb4 code, just that
it's fairly easy to do it as a separate step instead.

//Magnus