df: prioritize mounts nearer the device root

df: prioritize mounts nearer the device root

In the presence of bind mounts of a device, the 4th "mount root" field
from /proc/self/mountinfo is now considered, so as to prefer mount
points closer to the root of the device.  Note on older systems with
an /etc/mtab file, the source device was listed as the originating
directory, and so this was not an issue.
Details at http://pad.lv/1432871

* src/df.c (filter_mount_list): When deduplicating mount entries,
only prefer sources nearer or at the root of the device, when the
target is nearer the root of the device.
* NEWS: Mention the change in behavior.