[help needed] wc : fix issue mentionned in 4f043ff

[anastygnome]

This fixes the issue mentionned in the above commit merged in #3688, and ajusts one test accordingly. namely :

GNU wc will not align the output columns (compute_number_width() -> 1) in the specific case of the input for --files0-from=- being a named pipe, not real stdin. This difference can be triggered using the following two invocations.

• wc --files0-from=- < files0 => use a named pipe, GNU does align
• cat files0- | wc --files0-from=- => use real stdin, GNU does not align.

This is a crude attempt and needs attention ;)