add event for delivery of signal to app

This would be a convenience event to simplify certain types of tools. DR's main signal event is called on initial receipt of any signal, regardless of whether it the app has a register handler or not. For tools that want to know when an app signal handler is actually invoked, today they must monitor the sigaction syscalls themselves (Dr. Memory does this). Reasons that tools need to do this include updating stack shadows (Dr. Memory) or treating the signal like a call so that its return instruction, targeting the restorer code, is not an unmatched return.

Xref longstanding #241 (closed) on Windows about adding "asynchronous control change" events, which this could be folded into.