disable GnuTLS insecure ciphers

Make GnuTLS old and new consistent, specify the desired protocol, cipher
and certificate type in always in both modes. Disable insecure ciphers
as reported by howsmyssl.com. Honor not only --sslv3, but also the
--tlsv1[.N] switches.

Related Bug: http://curl.haxx.se/bug/view.cgi?id=1323