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/var.c

@@ -455,7 +455,7 @@ ParameterError setvariable(const char *input)
     if(use_stdin)
       file = stdin;
     else {
-      file = fopen(line, "rb");
+      file = curlx_fopen(line, "rb");
       if(!file) {
         errorf("Failed to open %s: %s", line, strerror(errno));
         err = PARAM_READ_ERROR;
@@ -469,7 +469,7 @@ ParameterError setvariable(const char *input)
     }
     curlx_dyn_free(&fname);
     if(!use_stdin && file)
-      fclose(file);
+      curlx_fclose(file);
     if(err)
       return err;
   }