sd-event: make sure RT signals are not dropped

sd-event: make sure RT signals are not dropped

RT signals operate in a queue, and we should be careful to never merge
two queued signals into one. Hence, makes sure we only ever dequeue a
single signal at a time and leave the remaining ones queued in the
signalfd. In order to implement correct priorities for the signals
introduce one signalfd per priority, so that we only process the highest
priority signal at a time.