This provides native ipv4 to clients on a network having an ipv6-only
backend, relying on external plat. This allows to use clients in
ipv6-only networks that would otherwise refuse to connect without a
valid ipv4 route.
Limitations:
* External plat must be provided, for example by jool - for example by
https://github.com/FreifunkMD/jool-docker.git
* This implementation will break ipv4 connections when clients roam.
