Signal handling.

[jmalloc]

Blocked by #85

Unlike streams, signal handlers need to be registered in an event-emitter like manner, otherwise signals could be missed if they arrive before the handler is re-registered (unless we always register handlers for all signals and buffer them, not sure of the performance impact there).

This could be accomplished with Recoil::callback() + pcntl_signal(), but it would be better if the kernel dispatched signals, probably when select() and equivalent is interrupted, as well as on a short-ish timer.