Handle NTFS timestamp, UNIX timestamp, Extended extra timestamp.
Writer supports only Extended extra timestamp field, matching most
zip creators.

Fixes #10242.

Change-Id: Id665db274e63def98659231391fb77392267ac1e
Reviewed-on: https://go-review.googlesource.com/18274
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>