Revert "libcontainer: Use pivot_root instead of chroot"

This reverts commit 5b5c884cc8266d0c2a56da0bc2df14cc9d5d85e8.

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

This reverts commit 757b5775725fb90262cee1fa6068fa9dcbbff59f.

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4512 from crosbymichael/no-pivot-root

No pivot root because of ramdisk