Cloud Foundry libcontainer API proposal

This builds on our [minimal changes](https://github.com/cf-guardian/libcontainer/commit/2575fbf353f9fab66c94fc476bec59805f8c288b) to [@vmarmol's API proposal](https://github.com/cf-guardian/libcontainer/commit/62e19761a077b3d5dfdd056df06da771b3a377ba).

This proposal is more ambitious and attempts to unify Containers with and without namespaces. A further possibility, not covered here, is to extract the non-namespaced "container" function as a "process in a control group" primitive.
We'd prefer to keep the API as simple as possible, so it would be good to avoid interpolation. A helper can be layered on top of the proposed API to deal with interpolation if it turns out to be a common requirement.