MFV r328220: 8677 Open-Context Channel Programs

MFV r328220: 8677 Open-Context Channel Programs

illumos/illumos-gate@a3b2868063897ff0083dea538f55f9873eec981f

https://www.illumos.org/issues/8677
  We want to be able to run channel programs outside of synching context.
  This would greatly improve performance of channel program that just gather
  information, as we won't have to wait for synching context anymore.

  This feature should introduce the following:
  - A new command line flag in "zfs program" to specify our intention to
  run in open context.
  - A new flag/option within the channel program ioctl which selects the
  context.
  - Appropriate error handling whenever we try a channel program in
  open-context that contains zfs.sync* expressions.
  - Documentation for the new feature in the manual pages.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>