Using subfs, users don't need to explicitly mount/unmount medium.
But there is no visual feedback, that medium is clean and can be unmounted, and
if application writes data randomly or slowly, there is a risk of writing while
medium is removed.

Proposal:

1) busy/clean signals will allow to notify user about device state,
- Implement subfs->HAL->DBUS busy/clean notification.
- Create busy emblem in Nautilus/Konqueror and update is using D-BUS signals.

2) clean_umount request will prevent further writing if user wants to unmount
the device.
- DBUS signal clean_umount will block subsequent subfs mounts.
- When umounted, signal is replied.
- Any subsequent access will fail.
- Nautilus/Konqueror will offer "Cleanly unmount" menu, which will issue
cleanly_umount signal to block subsequent writes until user removes the stick.