`sched_process_exit` tracepoint is called when thread terminates.
So exitsnoop shows line per each thread termination if the process
is multi-thread process. This is not useful when people wants to
know why process terminates, not thread.

So this changes exitsnoop default behavior which traces process termination
instead of thread termination. And add `--per-thread` option which behaves
as original exitsnoop implementation.