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)
I'd prefer return PAUSING and let the user retry. I have once gotten an infinite hang because there was a reclaimer kernel thread in the container. It should not happen for regular containers, but I don't want to rely on some kernel change hanging libcontainer and its caller.