Skip RAW kstat sysctls by default

hese kstats are often expensive to compute so we want to avoid them
unless specifically requested.

The following kstats are affected by this change:

kstat.zfs.${pool}.multihost
kstat.zfs.${pool}.misc.state
kstat.zfs.${pool}.txgs
kstat.zfs.misc.fletcher_4_bench
kstat.zfs.misc.vdev_raidz_bench
kstat.zfs.misc.dbufs
kstat.zfs.misc.dbgmsg

PR: 249258
Reported by: mjg
Reviewed by: mjg, allanjude
Obtained from: https://github.com/openzfs/zfs/pull/11099
Sponsored by: iXsystems, Inc.