This is needed for users on kernel older than 3.18 so they can avoid EBUSY
errors when trying to unlink, rename or remove a mountpoint that is present in
a shim namespace.
Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>
This also fix the type used for RuncOptions.SystemCgroup, hence introducing
an API break.
Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>
