change trace_end default functionality

Before:
trace_end means wait for interrupt(CTRL+C)

After:
just register ending function, that function will be called
when ktap script exit in last minute.

On the change, user could not to add trace_end in ktap script,
this change is good for one-line tracing script in future,

so the simplest tracing script just like be:
trace "syscalls:*" function (e) {
	printf("%d %d %s %s", cpu(), pid(), execname(), e.tostring);
}

But user can still use trace_end for some cleanup work, like
show histogram style.
kill some trace_end use