net/http/httptest: fix incorrectly-named ResponseRecorder example

R=golang-dev, minux.ma, r
CC=golang-dev
https://codereview.appspot.com/8545047
fmt: document that if String or Error panics, that value is printed

Fixes issue 5350.

R=r, minux.ma
CC=golang-dev
https://codereview.appspot.com/8950043
runtime: fix stack pointer corruption in runtime.cgocallback_gofunc()
runtime.setmg() calls another function (cgo_save_gm), so it must save
LR onto stack.
Re-enabled TestCthread test in misc/cgo/test.

Fixes issue 4863.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9019043