Make arc4random() depend on less subsystems by decoupling extract_entropy()

Make arc4random() depend on less subsystems by decoupling extract_entropy()
from the enqueue_randomness() logic.

Add add_event_data() which just enqueues some data into the event buffer
which can be used by extract_entropy(). On top of this remove the
timeout_del() call in dequeue_randomness() it does serve any meaningful
reason.
OK kettenis@ deraadt@ mpi@