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/inet_pton.c

@@ -112,7 +112,8 @@ inet_pton4(const char *src, unsigned char *dst)
 
     pch = strchr(digits, ch);
     if(pch) {
-      unsigned int val = *tp * 10 + (unsigned int)(pch - digits);
+      unsigned int val = (unsigned int)(*tp * 10) +
+                         (unsigned int)(pch - digits);
 
       if(saw_digit && *tp == 0)
         return (0);