Sectional zpool manpage

[overhacked]

Motivation and Context

As more subcommands have been added to zpool, the manpage has gotten to long to be easy to scroll through, especially on a text console. This PR is the completion of work started at the 2019 ZFS Dev Summit Hackathon. This PR is a companion to #9559.

Description

Most subcommands got their own manpages (e.g. create).
Some related commands grouped into a single manpage and
symlinks created (e.g. set & get). I did
this when topics were either too short to warrant their
own file or so interrelated that a user would want to
refer between commands in the same file.

Corrected .Sx internal references to .Xr cross refs

lots of .Sx references from when text was all in zpool.8
needed to be changed to .Xr zpool-$SUBCOMMAND 8 cross references.

Divided subcommand list in zpool(8) into sections

Divided into related functionality. This required writing new descriptions
for some commands.

How Has This Been Tested?

All altered manpages have been checked with mandoc -Tlint and return no warnings or errors.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #9564 into master will decrease coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9564      +/-   ##
==========================================
- Coverage   79.35%   79.05%   -0.31%     
==========================================
  Files         418      418              
  Lines      123686   123686              
==========================================
- Hits        98157    97778     -379     
- Misses      25529    25908     +379

Flag	Coverage Δ
#kernel	79.71% <ø> (-0.08%)	⬇️
#user	66.83% <ø> (-0.69%)	⬇️

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[Ornias1993]

Went through a big portion of it. See nothing to obviously wrong with this.

I think you can check off:

• Code cleanup (non-breaking change which makes code smaller or more readable)

• My code follows the ZFS on Linux code style requirements.

• All new and existing tests passed.

[overhacked]

Now that I've added SEE ALSO sections to the new subcommand manpages, I think that this PR could be ready for approval. If we can get a few more readers to make sure I haven't made any boneheaded errors, that'd be great. I'll mention that in the Slack channel.

[behlendorf]

The updated man pages look good to me as I read through them. I didn't spot any major issues or formatting problems. And I completely agree that once we get these big structural changes merged it'll be easier to tackle the other suggested improvements.

[overhacked]

@ahrens, could you sign off here also?