cryptui: Ensure a certificate's private key is exportable before allowing it to be selected for export.

cryptui: Skip password page when moving backward through the export wizard if it wasn't shown in the first place.

- A HWND can be safely marshaled over a LONG as its payload is not
  a pointer but a user handle.
- Use GetWindowLongPtr instead of GetWindowLong to retrieve a pointer.

Relaxing the tests on older versions of Windows caused a couple tests
to fail on more recent ones, which was unintended.

winebuild should be able to find them on its own.