OPTIM: stream_sock: save a failed recv syscall when splice returns EAGAIN

OPTIM: stream_sock: save a failed recv syscall when splice returns EAGAIN

When splice() returns EAGAIN, on old kernels it could be caused by a read
shutdown which was not detected. Due to this behaviour, we had to fall
back to recv(), which in turn says if it's a real EAGAIN or a shutdown.
Since this behaviour was fixed in 2.6.27.14, on more recent kernels we'd
prefer to avoid the fallback to recv() when possible. For this, we set a
variable the first time splice() detects a shutdown, to indicate that it
works. We can then rely on this variable to adjust our behaviour.

Doing this alone increased the overall performance by about 1% on medium
sized objects.