ZTS: reimplement kstat helper function

The old kstat helper function was barely used, I suspect in part because
it was very limited in the kinds of kstats it could gather.

This adds new functions to replace it, for each kind of thing that can
have stats: global, pool and dataset. There's options in there to get a
single stat value, or all values within a group.

Most importantly, the interface is the same for both platforms.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>