tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues fou

tests: make the unit test result type `CURLcode`

Before this patch, the result code was a mixture of `int` and
`CURLcode`.

Also adjust casts and fix a couple of minor issues found along the way.

Cherry-picked from #13489
Closes #13600

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build: fix some `-Wsign-conversion`/`-Warith-conversion` warnings

- enable `-Wsign-conversion` warnings, but also setting them to not
raise errors.
- fix `-Warith-conversion` warni

build: fix some `-Wsign-conversion`/`-Warith-conversion` warnings

- enable `-Wsign-conversion` warnings, but also setting them to not
raise errors.
- fix `-Warith-conversion` warnings seen in CI.
These are triggered by `-Wsign-converion` and causing errors unless
explicitly silenced. It makes more sense to fix them, there just a few
of them.
- fix some `-Wsign-conversion` warnings.
- hide `-Wsign-conversion` warnings with a `#pragma`.
- add macro `CURL_WARN_SIGN_CONVERSION` to unhide them on a per-build
basis.
- update a CI job to unhide them with the above macro:
https://github.com/curl/curl/actions/workflows/linux.yml -> OpenSSL -O3

Closes #12492

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tests: propagate errors in libtests

Use the test macros to automatically propagate some errors, and check
and log others while running the tests. This can help in debugging
exactly w

tests: propagate errors in libtests

Use the test macros to automatically propagate some errors, and check
and log others while running the tests. This can help in debugging
exactly why a test has failed.

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copyright: update all copyright lines and remove year ranges

- they are mostly pointless in all major jurisdictions
- many big corporations and projects already don't use them
- save

copyright: update all copyright lines and remove year ranges

- they are mostly pointless in all major jurisdictions
- many big corporations and projects already don't use them
- saves us from pointless churn
- git keeps history for us
- the year range is kept in COPYING

checksrc is updated to allow non-year using copyright statements

Closes #10205

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headers: reset the requests counter at transfer start

If not, reusing an easy handle to do a subsequent transfer would
continue the counter from the previous invoke, which then would mak

headers: reset the requests counter at transfer start

If not, reusing an easy handle to do a subsequent transfer would
continue the counter from the previous invoke, which then would make use
of the header API difficult/impossible as the request counter
mismatched.

Add libtest 1947 to verify.

Reported-by: Andrew Lambert
Fixes #9424
Closes #9447

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