dco: define OVPN_DEL_PEER_REASON_TRANSPORT_DISCONNECT on FreeBSD

dco: define OVPN_DEL_PEER_REASON_TRANSPORT_DISCONNECT on FreeBSD

FreeBSD's if_ovpn will never emit this as a peer deletion reason
(because it doesn't support TCP), but this allows us to align the
defines between Linux and FreeBSD, and remove a Linux-specific case from
process_incoming_del_peer().

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20230303110511.9569-1-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26324.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>