6866 zdb -l should return non-zero if it fails to find any label

illumos/illumos-gate@64723e361134b2a2c45341334174f9d34002f8d0
https://github.com/illumos/illumos-gate/commit/64723e361134b2a2c45341334174f9d34002f8d0

https://www.illumos.org/issues/6866

Need this for #6865.
To be generally more scripting-friendly, overload this issue with adding '-q'
option which should skip printing any label information.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>