glib/win32: introduce private g_win32_reopen_noninherited()

Used in following commits, including in some GIO experiments, so make it
a private API.

For now, this implementation is similar to the glib/gspawn-win32.c one,
with mroe error checking and better on error behaviour. A following
patch will also fix the case of duplicating sockets.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

glib/glib-private.h

@@ -171,6 +171,10 @@ typedef struct {
   /* See gwin32.c */
   gchar *(*g_win32_find_helper_executable_path) (const gchar *process_name,
                                                  void *dll_handle);
+
+  int                   (* g_win32_reopen_noninherited) (int      fd,
+                                                         int      mode,
+                                                         GError **err);
 #endif