GnuTLS improvement: gnutls_session_auto_verify_cert()

[Ted Zlatanov]

Recently, GnuTLS added the gnutls_session_auto_verify_cert() function to
simplify certificate verification. Due to lack of time, I can't work on
integrating it, but it seems like it will be a nice improvement over the
current fairly tedious code. It just needs to be conditional (for older
versions that don't have that function), so it will need a configure.ac
check.

So if anyone is interested in writing a small patch that will integrate
this new function, feel free to hack on it :)

Ted