Defining "container" and "container processes" on Linux

We'll need something like this for the signal/pause/resume actions
briefly referenced in the landed lifecycle [1].  See also the
discussion around stop action implementation [2].

[1]: https://github.com/opencontainers/specs/pull/231/files#diff-b84a8d65d8ed53f4794cd2db7e8ea731R48
[2]: https://github.com/opencontainers/specs/pull/225/files#r41900150

Author: wking