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)
Also, we are worried about the implications of the existing Config struct, which is why the CF proposal removed Name, Parent and Freezer from the cgroups portion of the configuration. A good API avoids providing multiple ways to do the same thing, so Name should not be part of the configuration. Parent seems to introduce a lot of implementation bias into the API, which would reduce the API's flexibility and increase its complexity. Freezer is pure duplication.