Pause/freeze containers during commit

Initiates a pause before committing a container,
adds a pause option to the commit command, defaulting to 'true'.

Fixes bug: #6267
Fixes bug: #3675

Docker-DCO-1.1-Signed-off-by: Eric Windisch <ewindisch@docker.com> (github: ewindisch)
updated docs & tests

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6787 from vieux/pause_commit2

Pause/freeze containers during commit