build: avoid overriding system symbols for fopen functions

By introducing wrappers for them in the curlx namespace:
`curlx_fopen()`, `curlx_fdopen()`, `curlx_fclose()`.

The undefine/redefine/`(function)()` methods broke on systems
implementing these functions as macros. E.g. AIX 32-bit's `fopen()`.

Also:
- rename `lib/fopen.*` to `lib/curl_fopen.*` (for `Curl_fopen()`)
  to make room for the newly added `curlx/fopen.h`.
- curlx: move file-related functions from `multibyte.c` to `fopen.c`.
- tests/server: stop using the curl-specific `fopen()` implementation
  on Windows. Unicode isn't used by runtests, and it isn't critical to
  run tests on longs path. It can be re-enabled if this becomes
  necessary, or if the wrapper receives a feature that's critical for
  test servers.

Reported-by: Andrew Kirillov
Bug: https://github.com/curl/curl/issues/18510#issuecomment-3274393640

Follow-up to bf7375ecc5 #18503
Follow-up to 9863599d69 #18502
Follow-up to 3bb5e58c10 #17827

Closes #18634

lib/curlx/fopen.h

@@ -0,0 +1,48 @@
+#ifndef HEADER_CURLX_FOPEN_H
+#define HEADER_CURLX_FOPEN_H
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+
+#include "../curl_setup.h"
+
+#include "multibyte.h"
+
+#if defined(_WIN32) && !defined(UNDER_CE)
+FILE *curlx_win32_fopen(const char *filename, const char *mode);
+#define CURLX_FOPEN_LOW(fname, mode) curlx_win32_fopen(fname, mode)
+#else
+#define CURLX_FOPEN_LOW fopen
+#endif
+
+#ifdef CURLDEBUG
+#define curlx_fopen(file,mode) curl_dbg_fopen(file,mode,__LINE__,__FILE__)
+#define curlx_fdopen(file,mode) curl_dbg_fdopen(file,mode,__LINE__,__FILE__)
+#define curlx_fclose(file) curl_dbg_fclose(file,__LINE__,__FILE__)
+#else
+#define curlx_fopen CURLX_FOPEN_LOW
+#define curlx_fdopen fdopen
+#define curlx_fclose fclose
+#endif
+
+#endif /* HEADER_CURLX_FOPEN_H */