Circpad: More/better pattern recognition events

Researchers might want other internal events for custom packet pattern recognition, so that they do not have to build quite so complicated state machines. One such example would be an event for ANY_BINS_EMPTY. Others might include packet arrival length counts or patterns.

These are best implemented by adding new circpad_internal_event_*() functions and internal event enums, so that it is easy for machines to make use of them.