os/signal, runtime: implement notes on Plan 9

os/signal, runtime: implement notes on Plan 9

This change is an implementation of the signal
runtime and os/signal package on Plan 9.

Contrary to Unix, on Plan 9 a signal is called
a note and is represented by a string.

For this reason, the sigsend and signal_recv
functions had to be reimplemented specifically
for Plan 9.

In order to reuse most of the code and internal
interface of the os/signal package, the note
strings are mapped to integers.

Thanks to Russ Cox for the early review.

Change-Id: I95836645efe21942bb1939f43f87fb3c0eaaef1a
Reviewed-on: https://go-review.googlesource.com/2164
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>