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

dsriram/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme