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>

At the same time, remove the "gen-certificate" target" since it is not
required as part of the build process and not documented elsewhere. This
script could be used for testing "examples/server".