Print helpful error message on --mktun/--rmtun if not available.

Print helpful error message on --mktun/--rmtun if not available.

OpenVPN only supports --mktun/--rmtun to create/destroy persistant
tunnels on Linux.  On BSD OSes, "ifconfig tun0 create" can do the
same job, so we do not actually need to support it - but the previous
error message ("unknown option") wasn't helpful.  So always accept
the option now, and on non-supported systems, direct user to manpage.

Trac #85

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <dazo@users.sourceforge.net>
Message-Id: <1430219063-12291-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9617
(cherry picked from commit 4ad2b65d9deb3197d847d7dcc36715aa5394836f)