zpool/zfs: allow --json wherever -j is allowed

Mostly so that with the JSON formatting options are also used, they all
look the same. To my eye, -j --json-flat-vdevs suggests that they are
different or unrelated, while --json --json-flat-vdevs invites no
further questions.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16632