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.