They might be used as special values in struct Timespec. See
http://man7.org/linux/man-pages/man2/utimensat.2.html for details.

Change-Id: I1c2d6d7731ca7f077d033e51ee6421a4f2cfbb89
Reviewed-on: https://go-review.googlesource.com/72376
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>