OpenZFS 6536 - zfs send: want a way to disable setting of DRR_FLAG_FREERECORDS

Authored by: Andrew Stormont <astormont@racktopsystems.com>
Reviewed by: Anil Vijarnia <avijarnia@racktopsystems.com>
Reviewed by: Kim Shrier <kshrier@racktopsystems.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6536
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/880094b