Remove the API container struct and use env

This change touches docker logs, attach, and start -a along with get
exit code in the client.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #6204 from crosbymichael/no-structs

Remove the API container struct and use env
Merge pull request #6205 from crosbymichael/rename-namespaces

Rename nsinit package to namespaces in libcontainer
Update nsinit to be nicer to work with and test
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)