Also add missing copyright headers with year determined from git log.

Change-Id: Iafc9881e746543f0a582dad2b0874d8399baf618
Reviewed-on: https://go-review.googlesource.com/122415Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>