perf top: Add --switch-on/--switch-off events

perf top: Add --switch-on/--switch-off events

Just like 'perf trace' and 'perf script', should be useful for instance
to only consider samples after the initialization phase of some
workload.

The man page has some examples and considerations about its current
interface, that still doesn't handle the on/off events in a special way,
behaving just like when multiple events are specified, i.e.:

- In non-group mode (when the event list is not enclosed in {}) show a
  a menu to allow choosing which event the user wants to see in the
  histograms browser

- In group mode, be it using {} or asking for --group, show one column
  per event.

Try for instance:

  # perf top -e '{cycles,instructions,probe:icmp_rcv}' --switch-on=probe:icmp_rcv

Replace probe:icmp_rcv, that I put in place using:

  # perf probe icmp_rcv:59

To hit when broadcast packets arrive, with a probe installed after an
initialization phase is over or after some other point of interest, some
garbage collection, etc, and also use --switch-off, for instance, on a
probe installed after said garbage collection is over.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: William Cohen <wcohen@redhat.com>
Link: https://lkml.kernel.org/n/tip-c7q7qjeqtyvc9mkeipxza6ne@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>