periodic/daily/801.trim-zfs: Add a daily zfs trim script

As mentioned in zpoolprops(7), on some SSDs, it may not be desirable to
use ZFS autotrim because a large number of trim requests can degrade
disk performance; instead, the pool should be manually trimmed at
regular intervals.

Add a new daily periodic script for this purpose, 801.trim-zfs. If
enabled (daily_trim_zfs_enable=YES; the default is NO), it will run a
'zpool trim' operation on all online pools, or on the pools listed in
'daily_trim_zfs_pools'.

The trim is not started if the pool is degraded (which matches the
behaviour of the existing 800.scrub-zfs script) or if a trim is already
running on that pool. Having autotrim enabled does not inhibit the
periodic trim; it's sometimes desirable to run periodic trims even with
autotrim enabled, because autotrim can elide trims for very small
regions.

PR: 275965
MFC after: 1 week
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/956

(cherry picked from commit 493908c4b45c7b75e4a6b4aea3b5b63ea5c268d5)