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_formparse.c

@@ -122,13 +122,13 @@ static struct tool_mime *tool_mime_new_filedata(struct tool_mime *parent,
     char *data = NULL;
     curl_off_t size;
     curl_off_t origin;
-    struct_stat sbuf;
+    curl_struct_stat sbuf;
 
     CURLX_SET_BINMODE(stdin);
     origin = ftell(stdin);
     /* If stdin is a regular file, do not buffer data but read it
        when needed. */
-    if(fd >= 0 && origin >= 0 && !fstat(fd, &sbuf) &&
+    if(fd >= 0 && origin >= 0 && !curlx_fstat(fd, &sbuf) &&
 #ifdef __VMS
        sbuf.st_fab_rfm != FAB$C_VAR && sbuf.st_fab_rfm != FAB$C_VFC &&
 #endif