cmd/buildlet/stage0: add untar mode

The new untar package will be shared by the buildlet later.

Change-Id: I7a33bf46e5cb6b7e5abd59613bb943039c84aa08
Reviewed-on: https://go-review.googlesource.com/41131
Reviewed-by: Jeff Johnson <jrjohnson@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed