OpenZFS 6866 - zdb -l non-zero status if no label

OpenZFS 6866 - zdb -l non-zero status if no label

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Don Brady <don.brady@intel.com>
OpenZFS-issue: https://www.illumos.org/issues/6866
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/3e4fae5
Closes #5730

Porting Notes:
- Omitted the illumos-specific `/dev/dsk` and `/dev/rdsk`
path conversions since they don't apply on linux.