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