man: use one Makefile, use OpenZFS for .Os

man: use one Makefile, use OpenZFS for .Os

The prevailing style is to use either nothing, or the originating
organisational umbrella (here: OpenZFS), and these aren't Linux manpages

This also deduplicates the substitution code, and makes adding/removing
sexions simpler in future

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12212