Add Windows DNS Leak fix using WFP ('block-outside-dns')

Add Windows DNS Leak fix using WFP ('block-outside-dns')

This option blocks all out-of-tunnel communication on TCP/UDP port 53
(except for OpenVPN itself), preventing DNS Leaks on Windows 8.1 and 10.

This is the same patch as dd628d2e0d786e4 in release/2.3, except that it
is always compiled (on WIN32) here - we already require compilation for
Vista+ in master (-> 2.4).

Reviewed-by: Selva Nair <selva.nair@gmail.com>
Reviewed-by: Lev Stipakov <lstipakov@gmail.com>
Reviewed-by: James Yonan <james@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1449780715-4027-1-git-send-email-iam@valdikss.org.ru>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10744

Signed-off-by: Gert Doering <gert@greenie.muc.de>