Improve specification of TYPE_SUB for event packets

TYPE_SUB is 4 bit, and for event packets we currently define three values for EV_LAST, EV_CONT, and EV_OVERFLOW. (c.f. https://opensocdebug.readthedocs.io/en/latest/02_spec/03_dataformats.html)

Plan:
- Have a single bit for split packets, i.e. EV_LAST and EV_CONT.
- Leave the three remaining bits for modul-specific tasks.
- Move EV_OVERFLOW into the spec of the individual modules.