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

Currently, the HTTPHeader constructor is templated with the types for
the header name and the value, but the actual name and value members are
of std::string type. It is currently not possible to have a different
type for the value for example.

This commit suggests removing the template for the constructor and
making the parameters to to be of type std::string instead.  I'd be
happy to look at a templated version but was not sure if it was worth
the effort as this is "just" in an example.

Correct addrlen comment for ngtcp_addr struct

This commit updates the comment for the addrlen to match the field
name change made in Commit 26440b08
("Rename len to addrlen").

Update CMakeLists.txt

OpenSSL embeds client certificate into ticket which might grow beyond
single QUIC packet limitation.  Technically, it is impossible for
OpenSSL based server and client to do resumption with client
certificate for now because QUIC draft mandates that the client
initial must be fit into a single Initial packet.

Embed ngtcp2_transport_params into ngtcp2_settings directly to avoid
conversion.

Original connection ID can now be specified directly to
ngtcp2_transport_params and can be passed to ngtcp2_conn_server_new
via ngtcp2_settings.  As a result, ngtcp2_conn_set_retry_ocid was
removed.

This commit removes the comment for a chunk argument for
ngtcp2_idtr_init as it does not seem to be relative to this function
declaration.