- 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
2dbe75bd7f