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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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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http: mark it 'this_is_a_follow' in the Location: logic

To make regular auth "reloads" to not count as redirects.

Verified by test 3101

Fixes #9885
Closes #9887