Implement listing on IPv4/IPv6 dual socket on all platform

With this patch OpenVPN will listen on Ipv4 as well as IPv6 when an IPv6
socket is used. Using bind ipv6only will disable this behavior
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1385382680-5912-7-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8052

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