The splice syscall is buggy prior to linux 2.6.29. Instead of returning
0 when reading a closed socket, it returns EAGAIN.  While it is possible
to detect this (HAProxy falls back to recv), it is simpiler to avoid
using splice all together. the "fcntl(fd, F_GETPIPE_SZ)" syscall is used
detect buggy versions of splice as the syscall returns EINVAL on
versions prior to 2.6.35.

Fixes #25486

Change-Id: I860c029f13de2b09e95a7ba39b76ac7fca91a195
Reviewed-on: https://go-review.googlesource.com/113999Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>