Update contrib/check-config.sh to check for CGROUP_FREEZER (which is newly required), and to use /proc/mounts for checking if we have a cgroup hierarchy instead of using /proc/$$/mountinfo (which on systemd might not list the cgroup hierarchy)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Avoid history expansion.

Docker-DCO-1.1-Signed-off-by: Jeff Welch <whatthejeff@gmail.com> (github: whatthejeff)
Improve documentation on Ubuntu for getting the latest release of
docker.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #6252 from tianon/update-check-config

Update contrib/check-config.sh to check for CGROUP_FREEZER and to use /proc/mounts
Merge pull request #6300 from whatthejeff/patch-1

Avoid history expansion.
Merge pull request #6315 from erikh/ubuntu-latest-install-docs

Improve documentation on Ubuntu for getting the latest release of docker.