This adds a crypto backend based on GnuTLS.  While most of the
gnutls_* functions used in this backend are officially available in
upstream GnuTLS, the following functions are only available in the
'tmp-quic' branch, for ABI assurance reasons until the QUIC standard
is finalized:
- gnutls_handshake_write
- gnutls_quic_get_peer_transport_params

Signed-off-by: Daiki Ueno <dueno@redhat.com>
Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>

Remove conn and user_data from encrypt/decrypt/hp_mask callbacks so
that they can be used without conn.

Fixes: https://github.com/ngtcp2/ngtcp2/issues/213

so that application is not bothered by another cryptic function.

* `ngtcp2_crypto_generate_stateless_reset_token` - Used to generate
  a stateless reset token as an HKDF extraction using the CID and
  a token secret as input.

Instead of installing key and iv with ngtcp2_conn_update_key, pass key
and iv buffers to ngtcp2_update_key and let application fill those
buffers.

When SSL_dl_handshake() returns either an SSL_ERROR_WANT_CLIENT_HELLO_CB
or SSL_ERROR_WANT_X509_LOOKUP, return specific error codes rather than
-1 so that those conditions can be differentiated and handled
appropriately