dco: implement dco support for p2mp/server code path

dco: implement dco support for p2mp/server code path

This change introduces ovpn-dco support along the p2mp/server code path.
Some code seems to be duplicate of the p2p version, but details are
different, so it couldn't be shared.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220805064555.13385-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24811.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>