* NEWS: Mention that df exits with nonzero status if it generates

* NEWS: Mention that df exits with nonzero status if it generates
no output.  This change was in 6.0 but inadvertently unmentioned.
* doc/coreutils.texi (df invocation): df exits nonzero if it outpus
nothing.
* src/df.c (file_systems_processed): Renamed from n_valid_args, and now
a boolean.
(show_dev): Don't set it until we actually output something.
Print the header if this is the first output.
(main): Don't print a header, as that is now show_dev's job.
* tests/misc/Makefile.am (TESTS): Add df.
* tests/misc/df: New file.