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

scripts/checksrc.pl

@@ -106,6 +106,7 @@ my %banfunc = (
     "fclose" => 1,
     "fdopen" => 1,
     "fopen" => 1,
+    "freopen" => 1,
     "open" => 1,
     "stat" => 1,
     );
@@ -961,7 +962,7 @@ sub scanfile {
         }
 
         # scan for use of non-binary fopen without the macro
-        if($l =~ /^(.*\W)(curlx_fopen|CURLX_FOPEN_LOW)\s*\([^,]*, *\"([^"]*)/) {
+        if($l =~ /^(.*\W)(curlx_fopen|CURLX_FOPEN_LOW|curlx_freopen|CURLX_FREOPEN_LOW)\s*\([^,]*, *\"([^"]*)/) {
             my $mode = $3;
             if($mode !~ /b/) {
                 checkwarn("FOPENMODE",