GnuTLS 2.10.0 added gnutls_certificate_set_verify_function() that can be
used to move peer certificate validation to an earlier point in the
handshake. Use that to get similar validation behavior to what was done
with OpenSSL, i.e., reject the handshake immediately after receiving the
peer certificate rather than at the completion of handshake.
Signed-off-by: Jouni Malinen <j@w1.fi>