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

man/man8/zfs-program.8

@@ -18,7 +18,7 @@
 .Nd executes ZFS channel programs
 .Sh SYNOPSIS
 .Cm "zfs program"
-.Op Fl n
+.Op Fl jn
 .Op Fl t Ar instruction-limit
 .Op Fl m Ar memory-limit
 .Ar pool
@@ -46,6 +46,10 @@ will be run on
 and any attempts to access or modify other pools will cause an error.
 .Sh OPTIONS
 .Bl -tag -width "-t"
+.It Fl j
+Display channel program output in JSON format. When this flag is specified and
+standard output is empty - channel program encountered an error. The details of
+such an error will be printed to standard error in plain text.
 .It Fl n
 Executes a read-only channel program, which runs faster.
 The program cannot change on-disk state by calling functions from the