Operating system counter stores signals as flag set without counter.
So sending two signals to a process may result to one or two signal
handler invocation. Queueing signals inside Idris could give users
false sense of signals being are queue, while they are not.
In particular, test for signal could not work reliably for that
reason.
Also, practically we usually don't need have more than once signal
event.
This is follow-up to #1660. CC @mattpolzin