Add zilstat script to report zil kstats in a user friendly manner

Added a python script to process both global and per dataset
zil kstats and report them in a user friendly manner similar
to arcstat and dbufstat.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #13704