Notifications for a signal.
fun ref apply(count: U32): Bool =>
Called with the the number of times the signal has fired since this was
last called. Return false to stop listening for the signal.
fun ref dispose() =>
Called if the signal is disposed. This is also called if the notifier
Raise a signal.
fun apply(sig: U32) =>
ifdef osx then
// On Darwin, @raise delivers the signal to the current thread, not the
// process, but kqueue EVFILT_SIGNAL will only see signals delivered to
// the process. @kill delivers the signal to a specific process.