CURLOPT_*-docs: provide additional details

Went through CURLOPTTYPE_STRINGPOINT and CURLOPTTYPE_SLISTPOINT options
and clarified:

- what happens when setting the option *again*

CURLOPT_*-docs: provide additional details

Went through CURLOPTTYPE_STRINGPOINT and CURLOPTTYPE_SLISTPOINT options
and clarified:

- what happens when setting the option *again*

- setting to NULL disables/restores to default

- libcurl does not copy the slist for options using a such

Closes #14846

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docs: fix some examples in man pages

The man-examples job hasn't been running for a few months to verify
that these compile, so some mistakes slipped in.

Ref: #14654

misc: cleanup after removing years from copyright

- remove leftover copyright years from few test files
- fix email in copyright lines
- consistent format of copyright lines

misc: cleanup after removing years from copyright

- remove leftover copyright years from few test files
- fix email in copyright lines
- consistent format of copyright lines

Closes #14312

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curldown: fixups

- make DEFAULT sections less repetitive

- make historic mentions use HISTORY

- generate the protocols section on `# %PROTOCOLS%` instead of guessing

curldown: fixups

- make DEFAULT sections less repetitive

- make historic mentions use HISTORY

- generate the protocols section on `# %PROTOCOLS%` instead of guessing
where to put it

- generate the availability section on `# %AVAILABILITY%` instead of
guessing where to put it

- make the protocols section more verbose

Closes #14227

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curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
wording

- allows us to double-check against other d

curldown: make 'added-in:' a mandatory header field

- generate AVAILABILITY manpage sections automatically - for consistent
wording

- allows us to double-check against other documumentation (symbols-in-versions
etc)

- enables proper automation/scripting based on this data

- lots of them were wrong or missing in the manpages

- several of them repeated (sometimes mismatching) backend support info

Add test 1488 to verify "added-in" version numbers against
symbols-in-versions.

Closes #14217

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CURLOPT_ECH.md: remove repeated 'if'

Closes #13922

CURLOPT_ECH: polish

- remove the pointer to build instructions, it won't work in manpages
- add see-also
- minor white space edits

Closes #13379

TLS: add support for ECH (Encrypted Client Hello)

An EXPERIMENTAL feature used with CURLOPT_ECH and --ech.

Closes #11922