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

tests/libtest/cli_h2_serverpush.c

@@ -30,7 +30,7 @@ static FILE *out_download;
 
 static int setup_h2_serverpush(CURL *hnd, const char *url)
 {
-  out_download = fopen("download_0.data", "wb");
+  out_download = curlx_fopen("download_0.data", "wb");
   if(!out_download)
     return 1;  /* failed */
 
@@ -72,7 +72,7 @@ static int server_push_callback(CURL *parent,
   curl_msnprintf(filename, sizeof(filename) - 1, "push%u", count++);
 
   /* here's a new stream, save it in a new file for each new push */
-  out_push = fopen(filename, "wb");
+  out_push = curlx_fopen(filename, "wb");
   if(!out_push) {
     /* if we cannot save it, deny it */
     curl_mfprintf(stderr, "Failed to create output file for push\n");
@@ -129,7 +129,7 @@ static CURLcode test_cli_h2_serverpush(const char *URL)
 
   easy = curl_easy_init();
   if(setup_h2_serverpush(easy, URL)) {
-    fclose(out_download);
+    curlx_fclose(out_download);
     curl_mfprintf(stderr, "failed\n");
     return (CURLcode)1;
   }
@@ -166,9 +166,9 @@ static CURLcode test_cli_h2_serverpush(const char *URL)
 
   curl_multi_cleanup(multi_handle);
 
-  fclose(out_download);
+  curlx_fclose(out_download);
   if(out_push)
-    fclose(out_push);
+    curlx_fclose(out_push);
 
   return CURLE_OK;
 }