Implement '--redirect-gateway ipv6'

Implement '--redirect-gateway ipv6'

Add "ipv6" and "!ipv4" sub-options to "--redirect-gateway" option.

This is done in the same way as in the OpenVPN 3 code base, so
"--redirect-gateway ipv6" will redirect both IPv4 and IPv6 - if you
want v6-only, use "--redirect-gateway ipv6 !ipv4".

The actual implementation is much simpler than for IPv4 - we just
add a few extra routes to the route_ipv6_option_list and leave it to
init_route_ipv6_list() to figure out whether there is an overlap with
IPv6 transport, and if yes, insert a host route to the VPN server
via the current IPv6 default gateway.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1441985627-14822-8-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10086