introduced gnutls_certificate_retrieve_function3

introduced gnutls_certificate_retrieve_function3

That allows a certificate callback to provide OCSP responses in addition
to certificates. That also introduces a flags option which currently
accepts GNUTLS_CERT_RETR_DEINIT_ALL which allows the callback to
specify whether the provided data should be deinitialized.

To simplify the certificate callback code, all previous (now legacy)
callbacks are implemented as wrappers over the new callback function.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>