Fix OVPN_DEL_PEER_REASON_TRANSPORT_DISCONNECT breakage on FreeBSD+DCO

Fix OVPN_DEL_PEER_REASON_TRANSPORT_DISCONNECT breakage on FreeBSD+DCO

commit 67c4eebdae introduces a new peer disconnect reason (transport
disconnected, aka "TCP session closed") which breaks compilation on
FreeBSD - OVPN_DEL_PEER_REASON_TRANSPORT_DISCONNECT not part of the
enum in freebsd_dco.h, and no kernel support for TCP anyway.

This patch is an intermediate bandaid, making the offending code in
multi.c "linux only" while a better solution is discussed.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20230113080745.82783-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/search?l=mid&q=20230113080745.82783-1-gert@greenie.muc.de
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit cf545d603ecd9fbacc6bd519efaa92d60f944287)