runtime: print "created by" for running goroutines in traceback
This allows to at least determine goroutine "identity".
Now it looks like:
goroutine 12 [running]:
        goroutine running on other thread; stack unavailable
created by testing.RunTests
        src/pkg/testing/testing.go:440 +0x88e

R=golang-dev, r, rsc
CC=golang-dev
https://codereview.appspot.com/12248043
strconv: fix doc comment for IntSize

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/12258043