Stub implementation of "--dhcp-option DNS6 <v6addr>"

Stub implementation of "--dhcp-option DNS6 <v6addr>"

This defines a new DHCP suboption "DNS6", but does not actually
implement anything but "document the option and understand it".

If received, it will be put into an "foreign_option_<n>" environment
variable where an --up script or plugin could receive and act upon it.

On non-Windows platforms, all "dhcp-option" sub-options end up there,
so v4 and v6 DNS options will be reflected like this:

   foreign_option_1=dhcp-option DNS6 2001:608::2
   foreign_option_2=dhcp-option DNS 195.30.0.2

v2: do not set o->dhcp_options if DNS6 is the single dhcp-option seen
    (spotted by Selva Nair)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <1479746562-751-1-git-send-email-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13174.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>