This function sets a callback to be called in order to retrieve the certificate
to be used in the handshake.
The callback's function prototype is:
int (*callback)(gnutls_session_t, const gnutls_datum_t* req_ca_dn, int nreqs,
const gnutls_pk_algorithm_t* pk_algos, int pk_algos_length, gnutls_retr_st* st);
req_ca_cert is only used in X.509 certificates.
Contains a list with the CA names that the server considers trusted.
Normally we should send a certificate that is signed
by one of these CAs. These names are DER encoded. To get a more
meaningful value use the function gnutls_x509_rdn_get().
pk_algos contains a list with server's acceptable signature algorithms.
The certificate returned should support the server's given algorithms.
st should contain the certificates and private keys.
If the callback function is provided then gnutls will call it, in the
handshake, after the certificate request message has been received.
The callback function should set the certificate list to be sent, and
return 0 on success. If no certificate was selected then the number of certificates
should be set to zero. The value (-1) indicates error and the handshake
will be terminated.