Illumos #1796, #2871, #2903, #2957

Illumos #1796, #2871, #2903, #2957

1796 "ZFS HOLD" should not be used when doing "ZFS SEND" from a read-only pool
2871 support for __ZFS_POOL_RESTRICT used by ZFS test suite
2903 zfs destroy -d does not work
2957 zfs destroy -R/r sometimes fails when removing defer-destroyed snapshot
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

References:
  https://www.illumos.org/issues/1796
  https://www.illumos.org/issues/2871
  https://www.illumos.org/issues/2903
  https://www.illumos.org/issues/2957

Ported by: Martin Matuska <martin@matuska.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>