container: update for changes to docker repo

Docker removed SetCloneFlags from `system`, and moved
the functionality to libcontainer.

https://github.com/docker/docker/commit/60341f80d7b7918d1c1eff7c38cbc55c9bdec4d9
agent: minor nits, constification, etc.
Merge remote-tracking branch 'origin/remove-docker-system-import' into agent-restart
agent: use channel naming style
agent: normalize agent/process state machine states