perf_counter tools: support symbolic event names in kerneltop

- kerneltop: --event_id => --event
- kerneltop: can accept SW event types now
- perfstat: it used to implicitly add event -2(task-clock),
	    the new code no longer does this. Shall we?

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>