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

[vszakats]

• 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

---

TODO:

• re-silence these warnings before merge.
• fix all -Wsign-conversion warnings. [FUTURE]
• delete CURL_WARN_SIGN_CONVERSION logic and delete -Wno-error=sign-conversion options. [FUTURE]

[vszakats]

-Wsign-conversion generates a bunch of warnings. These are made visible in this PRs CI runs.

E.g.:

• Linux: https://github.com/curl/curl/actions/runs/7235983469/job/19714040773?pr=12492
• Windows: https://ci.appveyor.com/project/curlorg/curl/builds/48774788/job/bunrtfj2awr2w7xl?fullLog=true

510-520 unique warnings in each, probably with large overlap.

There are a few options to proceed with this:

• Enable it with a -Wno-error exception, so that the warnings appear, but don't block. This might not play well with PowerShell in appveyor.ci, but it also has the undesired effect of polluting all logs with these warnings all the time, making actual errors difficult to spot and need searching for error: in them.
• Enable it in cmake / autotools, then silence it globally via #pragma in curl_setup.h, with an option to unsilence. This isn't useful on the get go, but allows working on these warnings locally, or in a dedicated CI job. This allows to move around the #pragma as ideally more and more code would have these warnings fixed, making it unnecessary at one point.
• Not enable this warning at all.

-Wsign-conversion is one of the few compiler hardening options suggested by OpenSSF:
https://best.openssf.org/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++#-Wsign-conversion