More IPv6-related updates to the openvpn man page.

Point to correct kernel version for --multihome and IPv4-mapped
addresses (3.15, Tore Anderson).

Remove old reference to http://www.greenie.net/ from the IPv6 section,
as the code and documentation in here is more current than on that site.
Some more additions and clarifications.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Tore Anderson <tore@fud.no>
Message-Id: <1398511854-3609-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8642