autorun: prefer Safe Removal over eject/unmount if possible

Basically do what NautilusPlacesSidebar does with the drive/volume/mount
eject/unmount/stop priorities.

We follow this pattern:
- always prefer Safely Remove if available (i.e. drive.stop())
- fallback to ejecting the mount/volume/drive if that's not possible
- finally, fallback to unmounting the mount if even eject is not
  available

This also means we don't care about the distinction between
Stop/Eject/Unmount at this level. Disk Utility (or Nautilus) are
available for those who want that degree of control, but the common case
here should do the most useful action without presenting the choice.

https://bugzilla.gnome.org/show_bug.cgi?id=653520