(#22665) mounts should autorequire parent mounts

Given two mount points:

    mount { "/var/log":
      # ...
    }

    mount { "/var":
      # ...
    }

The mount '/var/log' should autorequire '/var' so that the child mount
is not masked by the parent mount.
Merge pull request #1935 from adrienthebo/issue_22665-require_mount_parents

(#22665) mounts should autorequire parent mounts