Rewrite signal handling: use the self pipe

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Hi,

I noticied a race condition on FreeBSD when handling signals in asyncio when 
the Python main thread and a C thread are running at the "same time":
http://bugs.python.org/issue21645

I propose to not rely on signal.signal() to call 
_UnixSelectorEventLoop._handle_signal(), but use instead bytes written into the 
"wakeup fd" which is the loop "self" pipe.

Patch:
http://codereview.appspot.com/113160044

The patch changes the design of signal handling to fix the race condition:

1. C signal handler writes a byte in the "self" pipe
2. The event loop is awaken, reads its "self" pipe and schedules the call to 
the final signal handler
3. The signal handler is called

Original issue reported on code.google.com by victor.s...@gmail.com on 16 Jul 2014 at 3:52