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

docs/examples/.checksrc

@@ -11,6 +11,7 @@ allowfunc fdopen
 allowfunc fopen
 allowfunc fprintf
 allowfunc free
+allowfunc fstat
 allowfunc gmtime
 allowfunc localtime
 allowfunc malloc