When setting up TLS with cert-type OpenPGP from a client,

When setting up TLS with cert-type OpenPGP from a client,
the server verifies if it supports the extension’s contents
in _gnutls_session_cert_type_supported().  This function
checks for cred->get_cert_callback but not cred->get_cert_callback2.
As a result, servers setup for OpenPGP certificate credential
callback with gnutls_certificate_set_retrieve_function2() are
unable to use the OpenPGP certificate type.

The solution is to consider cred->get_cert_callback2 alongside
cred->get_cert_callback in _gnutls_session_cert_type_supported().

Patch by Rick van Rein.