Support IPv6 in rootless

Follow-up to:
- https://github.com/containerd/nerdctl/pull/1558

`rootlesskit --ipv6 --net=slirp4netns --port-driver=builtin` should just work, but `--port-driver=slirp4netns` depends on:
- https://github.com/rootless-containers/slirp4netns/pull/276
