* All sub filesystems (all datasets below the root fs dataset) will have
  mountpoint=legacy, to make them mountable on boot - only the root fs is
  mounted in the initrd.
* fs_create() now sets mountpoint=none by default, to make sure we
  can change it afterwards. It might fail if - when - the fs is
  mounted by the create but can't be unmounted when mountpoint is set.
  + Cleanup and false/true correction of fs_create().
* Add some support functions - create_disk() and create_partition()
  taken from partman-md. This will be needed when/if parted gets ZFS
  support (base idea is to have it 'fake' a dataset as a device).