Implement HMAC based session id for tls-crypt v2

Implement HMAC based session id for tls-crypt v2

Tls-crypt v2 is more complicated to implement a proper stateless
handshake. To allow state handshake this commit does

 - introduce a new packet CONTROL_WKC_V1 that repeats the wrapped
   client key.
 - introduce a way to negotiate the support for this packet in the
   three way handshake

Details about the protocol changes are in tls-crypt-v2.txt. Optional
arguments to the tls-crypt-v2 option have been added to explicitly
allow or disallow client that do not support the stateless handshake.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Patch v3: improve grammar, style, comments, fix unit tests
Patch v4: remove explicit flag for ability to resend WKc,
          clean up comments, improve code style in some instances
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20220505130348.1183195-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24287.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>