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

lib/curl_setup.h

@@ -28,6 +28,13 @@
 #define CURL_NO_OLDIES
 #endif
 
+/* FIXME: Delete this once the warnings have been fixed. */
+#if !defined(CURL_WARN_SIGN_CONVERSION)
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif
+#endif
+
 /* Set default _WIN32_WINNT */
 #ifdef __MINGW32__
 #include <_mingw.h>