Add weekly and monthly systemd timers for trimming

On machines using systemd, trim timers can be enabled on a per-pool
basis. Weekly and monthly timer units are provided. Timers can be
enabled as follows:

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

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

The manpage zpool-trim has been updated accordingly.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Julian Brunner <julian.brunner@gmail.com>
Closes #13544

etc/systemd/system/zfs-trim-monthly@.timer.in

@@ -0,0 +1,12 @@
+[Unit]
+Description=Monthly zpool trim timer for %i
+Documentation=man:zpool-trim(8)
+
+[Timer]
+OnCalendar=monthly
+Persistent=true
+RandomizedDelaySec=1h
+Unit=zfs-trim@%i.service
+
+[Install]
+WantedBy=timers.target

etc/systemd/system/zfs-trim-weekly@.timer.in

@@ -0,0 +1,12 @@
+[Unit]
+Description=Weekly zpool trim timer for %i
+Documentation=man:zpool-trim(8)
+
+[Timer]
+OnCalendar=weekly
+Persistent=true
+RandomizedDelaySec=1h
+Unit=zfs-trim@%i.service
+
+[Install]
+WantedBy=timers.target

etc/systemd/system/zfs-trim@.service.in

@@ -0,0 +1,15 @@
+[Unit]
+Description=zpool trim on %i
+Documentation=man:zpool-trim(8)
+Requires=zfs.target
+After=zfs.target
+ConditionACPower=true
+ConditionPathIsDirectory=/sys/module/zfs
+
+[Service]
+EnvironmentFile=-@initconfdir@/zfs
+ExecStart=/bin/sh -c '\
+if @sbindir@/zpool status %i | grep -q "(trimming)"; then\
+exec @sbindir@/zpool wait -t trim %i;\
+else exec @sbindir@/zpool trim -w %i; fi'
+ExecStop=-/bin/sh -c '@sbindir@/zpool trim -s %i 2>/dev/null || true'