Use auto-freq events by default in perf record and
perf top.

This allows more consistent hardware event sampling,
regardless of the intensity of the underlying event.

It also keeps us from over-sampling on larger/busier
systems.

(also make surrounding initializations more consistent)

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>