os: don't allow Process.Kill after Process.Release
os: don't allow Process.Kill after Process.Release

This is a user error, but killing -1 kills everything, which
is a pretty bad failure mode.

Fixes Issue 7434

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/70140043

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang