pkg/runtime: Plan 9 signal handling in Go

pkg/runtime: Plan 9 signal handling in Go

This adds proper note handling for Plan 9,
and fixes the issue of properly killing go procs.
Without this change, the first go proc that dies
(using runtime·exit()) would kill all the running
go procs. Proper signal handling is needed.

R=golang-dev, ality, rminnich, rsc
CC=golang-dev, john, mirtchovski
https://golang.org/cl/5617048