RFC: df --output=name to show specified file names

[Pádraig Brady]

I was looking at disk usage today and found that being able
to output the specified file names in the display would be useful.
Consider the new 'Name' column in:

$ df --o=target,name,pcent /var/*
Mounted on Name          Use%
/          /var/cache     71%
/          /var/lib       71%
/          /var/local     71%
/run       /var/lock       1%
/          /var/log       71%
/run       /var/run        1%
/          /var/spool     71%
/var/tmp   /var/tmp       71%
/          /var/www       71%

The attached implements this.
Worth adding?

One caveat is that the Name is '-' if you don't specify any arguments to df,
so therefore if you output all columns by using --output without a parameter,
you'll get a redundant column. I suppose one might suppress this column in that 
case?

thanks,
Pádraig.

df-output-name.patch
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