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

docs/internals/CHECKSRC.md

@@ -76,7 +76,8 @@ warnings are:
 
 - `EXCLAMATIONSPACE`: space found after exclamations mark
 
-- `FOPENMODE`: `curlx_fopen()` needs a macro for the mode string, use it
+- `FOPENMODE`: `curlx_fopen()`, `curlx_freopen()` need a macro for the mode
+  string, use it
 
 - `INDENTATION`: detected a wrong start column for code. Note that this
    warning only checks some specific places and can certainly miss many bad