Large change greatly expanding the linked footprint of this little
utility, but it is much faster and adds a couple of very nice features:
- set container sysctl settings to disable router advertisements as part
of container definition. this means we no longer need to do this via
`ip netns exec <container> sysctl -w ...` (followed by
`ip netns exec ip -6 a flush...`). Major win.
- able to very quickly ascertain creation and run state of defined
containers.
