zfsd: Use vdev prop values for fault/degrade thresholds

ZED uses vdev props for setting disk fault/degrade thresholds, this
patch enables zfsd to use the same vdev props for these same tasks.

OpenZFS on Linux is using vdev props for ZED disk fault/degrade
thresholds. Originally the thresholds supported were for io and checksum
events and recently this was updated to process slow io events as
well, see
https://github.com/openzfs/zfs/commit/cbe882298e4ddc3917dfaf239eca475fe06d62d4

This patch enables us to use the same vdev props in zfsd as ZED uses.
After this patch is merged both OSs will use the same vdev props to set
retirement thresholds.

It's probably important to note that the threshold defaults are
different between OS. I've kept the existing defaults inside zfsd and
DID NOT match them to what ZED does.

Differential Revision: https://reviews.freebsd.org/D44043
MFC after: 2 weeks
Relnotes: yes
Reviewed by: asomers, allanjude
Sponsored by: Axcient
Submitted by: Alek Pinchuk <apinchuk@axcient.com>