build: tidy up curl-specific fstat calls and stat struct type

To avoid redefining the `fstat` system symbol, and to clarify
`struct_stat` is a curl symbol.

- introduce `curlx_fstat()` macro and use it.
- rename `struct_stat` to `curl_struct_stat`.

Also:
- tests: replace direct `curlx_win32_stat()` call with `curlx_stat()`.
- checksrc: disallow direct `_fstati64` and `fstat()` calls, except in
  examples.

Closes #20496

src/tool_getparam.c

@@ -2209,7 +2209,7 @@ static ParameterError existingfile(char **store,
                                    const struct LongShort *a,
                                    const char *filename)
 {
-  struct_stat info;
+  curl_struct_stat info;
   if(curlx_stat(filename, &info)) {
     errorf("The file '%s' provided to --%s does not exist",
            filename, a->lname);