Enable tests for splice on Linux. Add a static (vl, I hate this)

Enable tests for splice on Linux. Add a static (vl, I hate this)
so we can detect broken Linux recvfile splice and correctly fall
back.
Karolin, I think this should be pulled into 3.2-stable even
though it's a feature change as otherwise there's no way for
3.2 to use recvfile zerocopy, which is an advertised feature.
Others please comment.
Jeremy.