NAT66 so non-cjdns client devices on the AP can connect to Hype sites

[benhylau]

Install radvd and in /etc/radvd.conf:

interface wlan0 {
    AdvSendAdvert on;
    MaxRtrAdvInterval 30;
    prefix fdfc::/64
    {
        AdvOnLink on;
        AdvAutonomous on;
    };
};

See: https://interfect.github.io/#!/posts/003-Meshnet-Captive-Portal.md