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_doswin.c

@@ -291,7 +291,7 @@ static SANITIZEcode rename_if_reserved_dos(char ** const sanitized,
    * to rename such files. */
   char *p, *base, *buffer;
 #ifdef MSDOS
-  struct_stat st_buf;
+  curl_struct_stat st_buf;
 #endif
   size_t len, bufsize;