Use EVFILT_SIGNAL for kqueue-enabled systems

[notgull]

kqueue supports EVFILT_SIGNAL, which would allow us to wait on a signal without needing to use a pipe. I'm working on exposing this functionality in async-io via smol-rs/async-io#112. However, this would require setting up an executor to listen for signals.

[notgull]

Not really needed and it might have surprising behavior that I don't want to deal with.