os: point users of ProcessState.SysUsage to getrusage(2)
os: point users of ProcessState.SysUsage to getrusage(2)

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7352044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go