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

scripts/checksrc.pl

@@ -49,6 +49,7 @@ my @ignore_line;
 
 my %banfunc = (
     "_access" => 1,
+    "_fstati64" => 1,
     "_lseeki64" => 1,
     "_mbscat" => 1,
     "_mbsncat" => 1,
@@ -82,6 +83,7 @@ my %banfunc = (
     "free" => 1,
     "freeaddrinfo" => 1,
     "freopen" => 1,
+    "fstat" => 1,
     "getaddrinfo" => 1,
     "gets" => 1,
     "gmtime" => 1,