Links v2: Execin & Ethernet move

This is a part of linksv2. This provides two sets of coupled functionality. `execin` provides a way to execute operations within a namespace (via libcontainer) and `MoveEthernetDevice` is hopefully self-explanatory.

I don't currently have tests because the bits required to get integration-cli to play well are not there yet. So, please do not merge, but review carefully. Thanks!
I think this is not very usable without `reexec` things, so possibly should be merged to `links` or new package as I proposed earlier.