This function makes the program stop for s seconds.
Only signal handlers can interrupt the sleep, and only when
abort_on_signal is set. If more than one thread is running
the signal must be sent to the sleeping thread. Other callbacks
are not called during sleep.
If s is zero then this thread will yield to other threads but
not sleep otherwise. Note that Pike yields internally at regular
intervals so it's normally not necessary to do this.
sizeof()