Improve help/usage (-h, -H) formatting

[jonpalmisc]

Per our brief discussion in #3383, I've gone ahead and re-formatted the CLI help output. For convenience, I've attached text files containing the before/after of the help output.

Some main points with respect to the help formatting are:

• Flag/option descriptions now use proper capitalization and punctuation.
• Flag/options descriptions are now consistently aligned and formatted.
• Some non-technical references to 'zstd' have be rewritten to 'Zstandard'; the rationale here is that this output is for humans, and in instances where the compression algorithm rather than the CLI tool itself is being referred to, I think this makes sense.
• Some flag/option descriptions have been rephrased slightly (or significantly) to what I think is better phrasing. Once again, this is opinionated and I'm happy to discuss changes.
• Probably some more things I'm forgetting.

I've also cleaned up the "welcome message" a bit in a separate commit; I expect this is an opinionated change and I did it in a separate commit so it can be easily reverted.

Please let me know your thoughts.

ZstdAfter.txt
ZstdBefore.txt

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[yoniko]

Hi @jonpalmisc,
I've only given it a brief look but it looks great from what I've seen.
Please note that there's a test that checks the help message and it's failing, so this PR will need to make the same changes to the expected help message in the tests, please see basic/help.sh* files for the test's details.

[Cyan4973]

I agree, this is a good update.

@jonpalmisc , if you have issues with the testing logic (which must be fixed before merge),
I could also take over from here and modify the tests so they correspond to the new output format.

[jonpalmisc]

Hey guys, thanks for the feedback; glad to hear these changes are a step in the right direction. I'll take a look at fixing that test.

[jonpalmisc]

Looks to me like those two tests that were failing before pass now. Please let me know if you spot any other issues.

I see the 'versions-compatibility' test is still failing but it seems like that is failing on most recent PRs; given that wasn't mentioned earlier, I assume that's a known/irrelevant issue?

[Cyan4973]

Yes, that's unrelated.
Thanks @jonpalmisc !