Push server mtu to client when supported and support occ mtu

Push server mtu to client when supported and support occ mtu

To maximise compatibility allow to lie our MTU in the default OCC
message.

Patch v2: improve documentation
Patch v3: split changing default MTU into its own patch
Patch v5: remove leftover mentions to default MTU

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221109154810.1268403-2-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25499.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>