freebsd/kstat: allow multi-level module names

This extends the existing special-case for zfs/poolname to split and
create any number of intermediate sysctl names, so that multi-level
module names are possible.

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