LGTM=bradfitz R=rsc, bradfitz CC=golang-codereviews https://golang.org/cl/167190043
Fixes golang/go#8508. LGTM=bradfitz R=bradfitz CC=golang-codereviews https://golang.org/cl/128920043
Clear signed messages are often seen in email and are formatted so that the plaintext of the signed message is readable without any special tools. R=golang-dev, bradfitz CC=golang-dev https://golang.org/cl/5604043