df: should error on over-mounted device, but doesn't

When two devices are mounted at the same directory and the earlier one is given as a positional argument to `df`, then it should print to stderr the message:
```
df: cannot access '/dev/loop1': over-mounted by another device
```

uutils df does not do this, but GNU df does.

This corresponds to GNU test suite file `tests/df/over-mount-device.sh`.