While we're here, fix the implementation of Release on both
Unix and Windows: Release is supposed to make Signal an error.

While we're here, make sure we never Signal pid 0.
(Don't try this at home.)

Fixes #7658.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://golang.org/cl/152240043