If the initial handshake is incomplete and the server sends a
no_renegotiation alert, the client should treat it as a fatal error
even if its level is warning.  Otherwise the same handshake
state (e.g., DHE parameters) are reused in the next gnutls_handshake
call, if it is called in the loop idiom:

  do {
          ret = gnutls_handshake(session);
  } while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

Signed-off-by: Daiki Ueno <ueno@gnu.org>