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)
The possibility of a "system error" being returned gives the impression that the returned Config accurately reflects the underlying system state. This seems to me to be much more useful than simply handing the user back the last Config she passed in (that facility could easily be layered on top of a libcontainer API by anyone who needed it, so I think we should excise it from the core).