Also add the SFD_* constants and SignalfdSiginfo type.

Change-Id: I3415c212b85c9d5a940ba2bf70f43739a2630d78
Reviewed-on: https://go-review.googlesource.com/c/157897
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>