Revert to using the upstream zpool-features(7) man page

[allanjude]

Summary

In the past, when it didn't look like ZFS was going to continue when Oracle closed it down, mm@ forked the man pages and rewrote them in mdoc(7).

While the rewritten man pages had nicer and more symantically rich markup, the divergence causes additional work every time a new feature is imported.

As an example rS337669 imported the large_dnode feature from Linux but missed the man page update (likely because Linux stores the man page in a different location)

There are no changes to the text of the man page, except to correct the chapter markers, and remove a comment about FreeBSD's boot loader not supporting large_blocks (it does now)

Once this is committed, I'll import the large_dnode feature section from ZoL.

The inspiration for this was was the site https://zgrep.org/zfs.html which parses this man page to determine which features are supported by which OSes, and it incorrectly indicates that large_dnode is not supported by 12.0 and 13-current.

[avg]

Unfortunately, I disagree with this change. I think that it is backwards.
illumos has already translated their zfs.1m and zpool.1m to mdoc format, so I assume that that's the direction they are moving in.
I'd rather help them with that in this case as well.
IMO, that would be a better way to overcome the divergence.

[allanjude]

In D18494#394133, @avg wrote:

Unfortunately, I disagree with this change. I think that it is backwards.
illumos has already translated their zfs.1m and zpool.1m to mdoc format, so I assume that that's the direction they are moving in.
I'd rather help them with that in this case as well.
IMO, that would be a better way to overcome the divergence.

I see that now, seems they just never got around to doing it for the zpool-features man page.

I'll do that and coordinate upstreaming it to illumos and ZoL.

[gbe]

Speaking with the manpages hat on, is this still an issue? As far as I remember we're about to import OpenZFS soon, which brings also the updated manual pages from upstream.