fix remount because of a different device name

- Compare real device path instead of given to those listed by mount (fix #8736)
- Compare given device name to those given by mount -l (#8725)
report umount because of different device

- Fails if the wrong device couldn't be unmounted
- Reports unmounting the wrong device
unmount comments
wanted and current devices comparison and display

- only get realpath of wanted device if it is a local path (begins with '/')
- build a current devices list for simple comparison and display
- display wanted (+ realpath) and current devices if different (changes)
- use active[name] construction instead of active.__getitem__(name):
  http://docs.python.org/2/reference/datamodel.html#special-method-names
get device list only if mounted
Merge pull request #9115 from dr4Ke/mount_device_realpath

mount state: better handling of wanted and current device