Fix '--dev null'

Fix '--dev null'

To test whether a server is reachable and all the key handling is
right, openvpn can connect with "--dev null --ifconfig-noexec" to
avoid needing to the client with elevated privileges.

This was erroring out for no good reason (because the "set environment
variables appropriately" code didn't know if this is a tun or tap
device...) - treat --dev null as "tap", done.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170224135222.44640-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14186.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 22c5381b71710ad0e1dbbccc1d5680fccb602311)