For people untarring with -p or as root, preserving file permissions.
This way we don't make tars owned by adg/eng or adg/staff or whatever
machine Andrew was on. Instead, we always build tarballs owned by predictable
users.

Except archive/tar doesn't seem to work.

Updates #3209.

R=golang-dev, adg
CC=dsymonds, golang-dev
https://golang.org/cl/5796064