Add DDT prune command

Requires the new 'flat' physical data which has the start
time for a class entry.

The amount to prune can be based on a target percentage of
the unique entries or based on the age (i.e., every entry
older than N days).

Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #16277

man/Makefile.am

@@ -72,6 +72,7 @@ dist_man_MANS = \
 	%D%/man8/zpool-create.8 \
 	%D%/man8/zpool-destroy.8 \
 	%D%/man8/zpool-detach.8 \
+	%D%/man8/zpool-ddtprune.8 \
 	%D%/man8/zpool-events.8 \
 	%D%/man8/zpool-export.8 \
 	%D%/man8/zpool-get.8 \