curlx: add and use curlx_freopen()

To complement the existing `curlx_fopen()` internal API.
It's used by the curl's `--stderr` option.

`curlx_freopen()` adds two features to the bare `freopen()`:
- tracing for debug-enabled builds.
- Unicode and long-filename support for Windows builds.

In effect this adds long-filename and enables Unicode support for
the `--stderr <filename>` curl command-line option on Windows.

Also add to checksrc.

Follow-up to 2f17a9b654 #10673

Closes #19598

src/tool_stderr.c

@@ -60,7 +60,7 @@ void tool_set_stderr_file(const char *filename)
   /* freopen the actual stderr (stdio.h stderr) instead of tool_stderr since
      the latter may be set to stdout. */
   /* !checksrc! disable STDERR 1 */
-  fp = freopen(filename, FOPEN_WRITETEXT, stderr);
+  fp = curlx_freopen(filename, FOPEN_WRITETEXT, stderr);
   if(!fp) {
     /* stderr may have been closed by freopen. there is nothing to be done. */
     DEBUGASSERT(0);