The qt_ntfs_permission_lookup mechanism is prone to data races

The qt_ntfs_permission_lookup is a mere int, but read by code that may execute in another than the GUI thread. The documentation (https://doc.qt.io/qt-6/qfiledevice.html#Permission-enum) is silent on the matter and doesn't mention that the flag needs to be upped before any other thread may be started which may perform filesystem I/O using Qt API, which, in any case, is very hard to follow for a user as it's unclear which threads Qt may start on its own.

Going forward, we should replace this with a RAII class or a pair of functions, and unexport the variable. For older Qt versions, we should add a warning to the docs.