Add ability to pause/unpause containers via cgroups freeze

This patch adds pause/unpause to the command line, api and drivers
for use on containers.  This is implemented using the cgroups/freeze
utility in libcontainer.

Co-Authored-By: Eric Windisch <ewindisch@docker.com>
Co-Authored-By: Chris Alfonso <calfonso@redhat.com>
Docker-DCO-1.1-Signed-off-by: Ian Main <imain@redhat.com> (github: imain)