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

lib/vssh/libssh2.c

@@ -1089,7 +1089,7 @@ static CURLcode ssh_state_pkey_init(struct Curl_easy *data,
       /* To ponder about: should really the lib be messing about with the
          HOME environment variable etc? */
       char *home = curl_getenv("HOME");
-      struct_stat sbuf;
+      curl_struct_stat sbuf;
 
       /* If no private key file is specified, try some common paths. */
       if(home) {