attach handling of signal arriving between handler being instated and takeover

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on July 29, 2013 18:20:12_

For attaching and taking over existing threads, there will always be a race
between installing our signal handler and sending the suspend signal to
each target native thread. We need to handle app signals received in a
native thread during this window.

If delayable, we can queue them up -- if our queue heap is set up.

If an itimer, we could just drop the signal.

If un-delayable, we may need to copy to the app's signal stack and
implement a go-native path to the app handler.

Xref issue #26 , issue #359 , issue #777 , which I believe cover different time windows
(pre-dcontext-setup).

Xref loosely related issue #150 .

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1234_