Use the client hello session identifier to transmit the client identifier

Use the client hello session identifier to transmit the client identifier

Currently the openconnect (protocol) client uses a custom extension to provide
information to the server on which session it was previously associated with.
However, a private extension cannot be defined in IETF without going through
a tedious standardization process involving the TLS working group. To avoid
that process we should provide the client identifier on the DTLS session using
alternative methods.

In TLS 1.3 (and DTLS) the session ID field was made obsolete, and as such we can
use it to place the client identifier instead of an extension field. We can do it
safely because (1) there is no session resumption -in the dtls1.2 or earlier sense-
and (2) ocserv is already checking this field for that value due to the old protocol
format.

Resolves #5

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>