Use only one splice if stdout is already a pipe

[dzamlo]

stdout is often already a pipe. In this case it's possible to directly splice to it, without an intermediate pipe.

It may be interesting to compare that with the current implementation. I wouldn't expect much difference, but it may be interesting nonetheless.

[mre]

Yup, would be a nice test.
I think the way to go would be to check each input argument if it already is a pipe. No idea if there's an easy check for that.