Use namespace.Exec() and namespace.Init() to execute processes in CT

This series is the second step to support the new API. Now an init process is actually executed in a new container. This series doesn't change code about creating containers, it only reworks code according with new API.

We are not going to save backward compatibility with old API, aren't we?
Yes, we will. @vmarmol has said the same. We can do a hack like we do in namespaces/nsenter.