Handle --dhcp-option DNS6 on Windows using netsh

Handle --dhcp-option DNS6 on Windows using netsh

v2: On closing tun delete the ipv6 dns addresses (if any were set).
Also use "validate=no" only in Windows 7 and higher where it is
supported. Its used to skip the time consuming automatic address
validation which is on by default on those platforms.

Tested on Windows Server 2008 (i686), Win 7 (x64) and Win 10 (x64)

TODO: set dns servers using the interactive service

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1479784332-21680-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13193.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>