cmd/5l: reestablish uniform union field naming
cmd/5l: reestablish uniform union field naming

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7226063
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go