This function sets a callback to be called when peers certificate
has been received in order to verify it on receipt rather than
doing after the handshake is completed. This overrides any callback
set using gnutls_certificate_set_verify_function().
The callbacks function prototype is:
If the callback function is provided then gnutls will call it, in the
handshake, just after the certificate message has been received.
To verify or obtain the certificate the gnutls_certificate_verify_peers2(),
gnutls_certificate_type_get(), gnutls_certificate_get_peers() functions
can be used.
The callback function should return 0 for the handshake to continue
or non-zero to terminate.