libsysdecode: add zfs iocs

From: Yuri <yuri_at_aetern.org>
Date: Tue, 18 Jul 2023 14:38:10 UTC
While looking into various zfs issues, I noticed that truss doesn't
decode zfs ioctl names which was somewhat unhelpful, so here's (a bit
ugly and hackish) fix for that, please review:

https://reviews.freebsd.org/D41063