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

src/tool_formparse.c

@@ -563,14 +563,14 @@ static int get_param_part(char endchar,
             endpos--;
         sep = *p;
         *endpos = '\0';
-        fp = fopen(hdrfile, FOPEN_READTEXT);
+        fp = curlx_fopen(hdrfile, FOPEN_READTEXT);
         if(!fp)
           warnf("Cannot read from %s: %s", hdrfile,
                 strerror(errno));
         else {
           int i = read_field_headers(hdrfile, fp, &headers);
 
-          fclose(fp);
+          curlx_fclose(fp);
           if(i) {
             curl_slist_free_all(headers);
             return -1;