Fix for issue 1161 ( --route-ipv6 does not recognize net_gateway or net_gateway-ipv6 )

[fkooman]

This is a potential fix for 1161. I am sure it is not done properly, some checks (whether iface / metric is defined) are missing I think, but rudimentary testing shows that it works on at least Linux. This patch adds/fixes the net_gateway_ipv6 flag for --route-ipv6.

Any feedback, hints, suggestions welcome before I send it to openvpn-devel@ list.