cmd/buildlet: add graceful shutdown on SIGTERM and SIGINT on Linux for GKE

Change-Id: If06cf0c1a8e2f78aa3822a2acbf99338a2be53f0
Reviewed-on: https://go-review.googlesource.com/33551
Reviewed-by: Kelsey Hightower <khightower@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed