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

tests/libtest/first.c

@@ -65,12 +65,16 @@ int select_wrapper(int nfds, fd_set *rd, fd_set *wr, fd_set *exc,
 
 void wait_ms(int ms)
 {
+  if(ms < 0)
+    return;
 #ifdef USE_WINSOCK
-  Sleep(ms);
+  Sleep((DWORD)ms);
 #else
-  struct timeval t;
-  curlx_mstotv(&t, ms);
-  select_wrapper(0, NULL, NULL, NULL, &t);
+  {
+    struct timeval t;
+    curlx_mstotv(&t, ms);
+    select_wrapper(0, NULL, NULL, NULL, &t);
+  }
 #endif
 }