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)
We really don't like an initial process being specified on Create().

If there is a PID namespace, this creates a fundamental asymmetry between the "created" process (which is then responsible for acting as an "init" process in the PID namespace) and any processes started using RunIn() or similar.

If there are no namespaces, then there's no reason to lump process creation in with Container creation.