df: don't fail due to an unreadable argument

df: don't fail due to an unreadable argument

* src/df.c (main): If open or fstat fails when we're trying to ensure
that all arg-partitions are automounted, fall back on using stat.
Inspired by the report and patch from Olivier Fourdan in
http://bugzilla.redhat.com/520630.
* NEWS (Bug fixes): Mention it.
* tests/df/unreadable: New test for the above.
* tests/Makefile.am (TESTS): Add df/unreadable.
The bug was introduced in coreutils-7.3 via commit dbd17157,
2009-04-28, "df: use open(2), not stat, to trigger automounting".