runtime: show m stack during crash on m stack

The various throwing > 0 finish a change started
in a previous CL, which sets throwing = -1 to mean
"don't show the internals". That gets set during the
"all goroutines are asleep - deadlock!" crash, and it
should also be set during any other expected crash
that does not indicate a problem within the runtime.

Most runtime.throw do indicate a problem within the
runtime, however, so we should be able to enumerate
the ones that should be silent. The goroutine sleeping
deadlock is the only one I can think of.

Update issue 5139

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13662043
cmd/yacc: replace units example with simpler expr example

The units example is nice but is covered by the Lucent
license, which may be a concern for some people making a
commercial source code distribution of Go.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13283045