df: implement the File column

[jfinkels]

Implement the "File" column in the df output table. Before this
commit, a blank entry appeared in the "File" column for each
row. After this commit, a "-" entry appears when df is run with no
positional arguments and the filename appears when run with positional
arguments. For example:

$ touch a b c && df --output=target,file a b c
Mounted on File
/          a
/          b
/          c

[sylvestre]

there is also this error:
missing field filein initializer ofRow``