Illumos #1377 `zpool status -D' should tell if there are no DDT entries

1337 `zpool status -D' should tell if there are no DDT entries

Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: George Wilson <gwilson@zfsmail.com>
Approved by: Albert Lee <trisk@nexenta.com>

References:
  illumos/illumos-gate@ce72e614c133351311e87bbbe4eba8fea9e77768
  illumos changeset: 13432:d1ad8d106d64
  https://www.illumos.org/issues/1337

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>