df: add --output=file to directly output specified arguments

df: add --output=file to directly output specified arguments

* src/df.c (usage): Document the new 'file' --output field.
(get_dev): Add a new parameter to pass the specified
argument from the command line through.  Use '-' if a
command line parameter is not being used.
* doc/coreutils.texi (df invocation): Describe the new 'file' field.
* tests/df/df-output.sh: Adjust all fields test, and
add a specific test for --output=file.
* NEWS: Mention the new feature.