close stdout/err handles before opening debug log

There is an unconfirmed report about file in use errors, most likely
the handles are inherited by some other process or an antivirus or
whatever, but lets at least close them in the under our control.

Windows is such a pain.

src/calibre/gui2/main.py

@@ -434,6 +434,8 @@ def run_gui_(opts, args, app, gui_debug=None):
         debugfile = runner.main.gui_debug
         from calibre.gui2 import open_local_file
         if iswindows:
+            # detach the stdout/stderr/stdin handles
+            winutil.prepare_for_restart()
             with open(debugfile, 'r+b') as f:
                 raw = f.read()
                 raw = re.sub(b'(?<!\r)\n', b'\r\n', raw)