Add the SOF_TIMESTAMPING_* flags used to request different types of
timestamping operations. In

  err := unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_TIMESTAMPING, val)

val is a mask of SOF_TIMESTAMPING_* values.

See https://www.kernel.org/doc/Documentation/networking/timestamping.txt
for more information.

Fixes golang/go#28173

Change-Id: I26e964d607606f2b3cb24f66f071d11765b9696e
Reviewed-on: https://go-review.googlesource.com/c/143337
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>