Similar to the tail -n0 patch recently sent to the list, this adjusts shuf so that it never reads input with -n0.
Now unlike tail -n0 which is useful with -f, shuf -n0 isn't needed, as it always truncates --output if specified, and doesn't produce anything. I don't think we should disallow -n0 though, as it could validly be used in scripts as a concise way to produce 0..N outputs. thanks, Pádraig
