Add support of utun devices under Mac OS X

Mac OS X 10.7+ natively supports tun devices (called utun). The "standard"
utun.ko driver is sometimes problematic (e.g. VmWare Fusion 5 and tun.ko
do not work together).

When OpenVPN is compiled with utun support it will if no dev-node is given
first try to use utun and if that is not available will try the
traditional tun devices

v2: Fixed tap support, get device name via ioctl, add manage
v3.1: Fix compiling without if/utun.h, fix manage errors
v4/v5: Don't try open to dynamically open utun0 -255 when early utun
initialization fails, fix fallback to tun, give fatal error message when
utun fails but no tun fallback should be done
v6: add commit message change log, replace strstr with strncmp, move
v7: Throw error if a user does the strange combination of --dev tun
--dev-type tap and --dev-node utun

A lot good input on earlier patches by Jonathan K. Bullard
<jkbullard@gmail.com>

Parts of the patches are inspired from Peter Sagerson's
<psagers@ignorare.net> utun patch
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Tested-by: Jonathan K. Bullard <jkbullard@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1371811708-8528-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7739
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fbc04bedbcce02fc625357b7475ddbc7164cabbf)