systemd: add weekly and monthly scrub timers

Timers can be enabled as follows:

systemctl enable zfs-scrub-weekly@rpool.timer --now
systemctl enable zfs-scrub-monthly@datapool.timer --now

Each timer will pull in zfs-scrub@${poolname}.service, which is not
schedule-specific.

Added PERIODIC SCRUB section to zpool-scrub.8.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Closes #12193

man/man8/zpool-scrub.8

@@ -116,8 +116,29 @@ scanned at 100M/s, and 68.4M of that file data has been
 scrubbed sequentially at 10.0M/s.
 .El
 .El
+.Sh PERIODIC SCRUB
+On machines using systemd, scrub timers can be enabled on per-pool basis.
+.Nm weekly
+and
+.Nm monthly
+timer units are provided.
+.Bl -tag -width Ds
+.It Xo
+.Xc
+.Nm systemctl
+.Cm enable
+.Cm zfs-scrub-\fIweekly\fB@\fIrpool\fB.timer
+.Cm --now
+.It Xo
+.Xc
+.Nm systemctl
+.Cm enable
+.Cm zfs-scrub-\fImonthly\fB@\fIotherpool\fB.timer
+.Cm --now
+.El
 .
 .Sh SEE ALSO
+.Xr systemd.timer 5 ,
 .Xr zpool-iostat 8 ,
 .Xr zpool-resilver 8 ,
 .Xr zpool-status 8