godoc: show source code if -src flag is set in command-line mode
godoc: show source code if -src flag is set in command-line mode

This also shows the source code of exported functions in server
mode (e.g. pkg/big/?m=src).

Fixes issue 2360.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5254057

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