Handle EBUSY on remount

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
* do not consider iptables' output an error in case of xtables lock
Docker-DCO-1.1-Signed-off-by: Giuseppe Mazzotta <gdm85@users.noreply.github.com> (github: gdm85)
Add ErrPrerequisites to improve misleading errors

There are two cases where we can't use a graphdriver:

1) the graphdriver itself isn't supported by the system
2) the graphdriver is supported by some configuration/prerequisites are
missing

This introduces a new error for the 2) case and uses it when trying to
run docker with btrfs backend on a non-btrfs filesystem.

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)
Merge pull request #6098 from crosbymichael/retry-mount

Handle EBUSY on remount
Merge pull request #6106 from discordianfish/improve-error-btrfs-on-non-btrfs

Add ErrPrerequisites to improve misleading errors
Merge pull request #6105 from gdm85/master

Do not consider iptables' output an error in case of xtables lock
Added a new method cgroups.GetStats() which will return a cgroups.Stats object which will contain all the available cgroup Stats.
Remove old Stats interface in libcontainers cgroups package.
Changed Stats to use unit64 instead of int64 to prevent integer overflow issues.
Updated unit tests.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)