Basic version of libcontainer API.

This version has mostly the behavior available today and the
functionality agreed upon in other PRs. Further changes will be sent as
smaller PRs for discussion.

This PR is meant to be the already agreed-upon components of the API (note that a lot of functionality is missing). The reviewing is getting harder so we'll try to get this in, and then make the smaller changes with more discussion in smaller PRs.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
If some processing are stuck in I/O, container state returned by freezer will be freezing. Are we going to report what freezer reports initially? I think it needs another call to check container state before we can move it to PAUSED. Are we planning to do retries?