Revert "Revert "libcontainer: Use MS_PRIVATE instead of MS_SLAVE""

This reverts commit bd263f5b15b51747e3429179fef7fcb425ccbe4a.

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Revert "Revert "libcontainer: Use pivot_root instead of chroot""

This reverts commit 82f797f14096430c3edbace1cd30e04a483ec41f.

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Add env var to toggle pivot root or ms_move

Use the  DOCKER_RAMDISK env var to tell the native driver not to use
a pivot root when setting up the rootfs of a container.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4519 from crosbymichael/back-to-pivot-root

Use env var to toggle pivot root or ms_move