Adjust Linux+FreeBSD DCO device name handling to 'non DCO linux style'

Adjust Linux+FreeBSD DCO device name handling to 'non DCO linux style'

On Linux, tun devices are created according to the following algorithm
  --dev tun    -> try tun0, tun1, ... tun255, use first free
  --dev anything -> create a TUN device named "anything"
(as long as "anything" is not "null" or "tap[N]")

DCO was following the "other platform convention", where everything
not having a digit was iterated ("--dev tun-home" -> "tun-home0") -
which does not work for classic tun/tap devices on the BSDs anyway,
so is not the best model.

Adjust open_tun_dco_generic() to document expected behaviour and
do the thing.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20220829190124.2636045-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25134.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>