Debian bug 528661: Disable some TLS extensions on GNUTLS.

Debian bug 528661: Disable some TLS extensions on GNUTLS.

- gnutls_handshake_set_private_extensions: Do not enable private cipher
  suites that might not be supported by anything other than GNUTLS.
  The GNUTLS 2.8.0 documentation notes that enabling these extensions
  can cause interoperability problems.
- gnutls_set_default_priority: Explicitly disable OpenPGP certificates.
- gnutls_certificate_type_set_priority: Do not enable OpenPGP certificates.
  The GNUTLS 2.8.0 documentation notes that OpenPGP certificate support
  requires libgnutls-extra.  Because libgnutls-extra 2.2.0 and later are
  under GPLv3-or-later and thus not GPLv2 compatible, ELinks doesn't use
  libgnutls-extra, so OpenPGP certificates didn't work anyway.
- gnutls_server_name_set: Do not tell the server the hostname from the URL.
  This was supposed to let the server choose the appropriate certificate
  for each name-based virtual host, but ELinks actually always sent just
  "localhost", so it didn't work anyway.  This will have to be revisited
  when ELinks is changed to actually verify the subject name from the
  server's certificate (ELinks bug 1024).

These changes should help ELinks negotiate SSL with bugzilla.novell.com.

[NEWS and commit message by me.  --KON]