JSON: Modify json_event() to use JSON_LAST

It would be more easier to use json_event() if We could indicate the argumentis
count by a special constant instead of a explicitly count argument(the old way).
Also change the 'EVENT_NONE' to 'JSON_LAST' in json_event_type.

* json.c(json_event, json_event_handler): Modify to use JSON_LAST as the
last argument of the variable argument list.
* defs.h: Modify the declaration of json_event()
* io.c, process.c, strace.c: Modify all reference of json_event() to adapt to
the new prototype.