FreeBSD-DCO: repair device iteration to find first free interface.

FreeBSD-DCO: repair device iteration to find first free interface.

During review/update phase, FreeBSD/DCO's ability to find the first
free tun interface on "--dev tun" got broken, due to two issues:

 - create_interface() called msg(M_ERR|...), which is a fatal error
   and aborts OpenVPN, so "no retry with 'tun1' after 'tun0' failed"

   Change to M_WARN|M_ERRNO (= warning level, add strerror(errno), return).

 - open_tun_dco_generic() expects "-errno" as return value of
   open_tun_dco(), and breaks the loop on -EPERM.  create_interface()
   was returning "-1" instead (ioctl() error signalling), which happens
   to be "-EPERM" on FreeBSD.

   Change create_interface() to return -errno.

While at it, remove logging of errors from dco_freebsd.c::open_tun_dco()
(because all errors from create_interface() would be already logged there),
reducing open_tun_dco() to just a wrapper around create_interface().

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Kristof Provost <kprovost@netgate.com>
Message-Id: <20220819182439.71531-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25034.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>