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

include/libnvpair.h

@@ -20,6 +20,7 @@
  */
 /*
  * Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013, Joyent, Inc. All rights reserved.
  */
 
 #ifndef	_LIBNVPAIR_H
@@ -46,6 +47,7 @@ extern int nvpair_value_match_regex(nvpair_t *, int, char *, regex_t *,
     char **);
 
 extern void nvlist_print(FILE *, nvlist_t *);
+int nvlist_print_json(FILE *, nvlist_t *);
 extern void dump_nvlist(nvlist_t *, int);
 
 /*