Add JSON output support to channel programs

Add JSON output support to channel programs

The changes piggyback JSON output support on top of channel programs
(#6558).  This way the JSON output support is targeted to scripting
use cases and is easily maintainable since it really only touches
one function (zfs_do_channel_program()).

This patch ports Joyent's JSON nvlist library from illumos to enable
easy JSON printing of channel program output nvlist.  To keep the
delta small I also took advantage of the fact that printing in
zfs_do_channel_program() was almost always done before exiting
the program.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #7281