misc/emacs: refine label detection
misc/emacs: refine label detection

Based on idea in
http://patch-tracker.debian.org/patch/series/view/golang/1:59-1/008-emacs-mode-key-literal-indent.patch

Fixes issue 2174.

R=amdragon, ajmani, amdragon
CC=golang-dev
http://codereview.appspot.com/4922049

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