Illumos 4373 - add block contents print to zstreamdump

4373 add block contents print to zstreamdump
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:

https://www.illumos.org/issues/4373
https://github.com/illumos/illumos-gate/commit/994fb6b

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