JSON: Refactor the interface of json_event

Redefine the json_event() interface to a macro, Rename all the EVENT_* to JSON_*.

* json.c(json_event, json_event_handler): rename json_event to json_output_event,
modify json_event_handler to eliminate use NEWLINE,SEPARATOR,BEFO_SEP.
* defs.h: redefine the interface of json_event, use __VA_ARGS__ as the
first argument of the json_event macro.
* io.c, process.c, strace.c syscall.c: modify all json_event to use the new interface.