MFV r354377: 10554 Implemented zpool sync command

MFV r354377: 10554 Implemented zpool sync command

illumos/illumos-gate@9c2acf00e275b6b2125a306f33cdddcc58393220
https://github.com/illumos/illumos-gate/commit/9c2acf00e275b6b2125a306f33cdddcc58393220

https://www.illumos.org/issues/10554
  During the port of MMP (illumos bug 10499) from ZoL, I found this
  earlier ZoL project is a prerequisite. Here is the original
  description.  This addition will enable us to sync an open TXG to the
  main pool on demand. The functionality is similar to 'sync(2)' but
  'zpool sync' will return when data has hit the main storage instead of
  potentially just the ZIL as is the case with the 'sync(2)' cmd.

Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Author: Alek Pinchuk <apinchuk@datto.com>
MFC after:	3 weeks
Relnotes:	possibly