df: use the last device name provided by the system

df: use the last device name provided by the system

The device name reported for a particular mount entry
may no longer be valid if the mount point was subsequently
mounted on a different device.  Therefore honor the order
of the mount list returned by the system and use the last
reported device name.

* src/df.c (filter_mount_list): When discarding the current
mount entry, ensure that a new device name is not also discarded.
* tests/df/skip-duplicates.sh: Add a test case.  Also fix
a false failure in the edge case of a system with only a
single file system.
* NEWS: Mention the fix.