Support for v6plus/DS-Lite static IP configuration

It seems that RAX120 and a lot of other NETGEAR routers have support for v6plus and DS-Lite, then adding a support for static IP configuration is technically really an easy part.

A static IP configuration on v6plus/DS-Lite networks needs to manually specify a few parameters, instead of gather from enabler.ne.jp automatically.

- For v6plus, they are v4IP address, Interface ID, BR address, User ID, Password;

- For DS-Lite, they are Target URL of DDNS Updating, Auth ID, Auth Password, Fully-Qualified Domain Name, DDNS ID, DDNS Password.

It should be really an easy work to add support for these configurations, but it will make us users who purchased a static IP from ISP greatly more convenient.