Client certificate authentication

Attached patch implements client certificate authentication.

I kept this sitting in my tree without sending it in before the
commitfest because it is entirely dependent on the
not-yet-reviewed-and-applied patch for how to configure client
certificate requesting. But now that I learned how to do it right in
git, breaking it out was very easy :-) Good learning experience.

Anyway. Here it is. Builds on top of the "clientcert option for pg_hba"
patch already on the list.

//Magnus