IPv6 WAN fail-over without IPv6 NAT

[jow]

@patrakov

All that is missing is the ability to set this dynamically to the single best WAN that is available at the moment and works - as opposed to a static list, or to the default of including everything.

Sounds like this functionality should be factored out into a link monitoring process which then dynamically drives this value.

Netifd could be extended with additional ubus procedures that allow modifying ip6class without the need to touch configuration.