dco: pass control packets through the socket on FreeBSD

dco: pass control packets through the socket on FreeBSD

FreeBSD allows packets to be sent through the socket even when the
if_dco driver is active, so prefer that path.

Also remove the FreeBSD dco_do_write() implementation, as this function
will never be called any more on FreeBSD. Assert this.

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221126090851.8656-1-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25542.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>