kqueue: try to extend signal handling to other platforms
kqueue: try to extend signal handling to other platforms

Clean up the preprocessor logic so that, hopefully, platforms other than
OpenBSD and Apple will work. Use sigtimedwait if _POSIX_REALTIME_SIGNALS
is > 0, otherwise fall back to setting up a signal handler.

davmac314/dasynq,davmac314/dasynq