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 <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Richard Elling <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alek Pinchuk <[email protected]>
Closes #7281 

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