perf stat: Implement control commands handling

Implement handling of 'enable' and 'disable' control commands coming
from control file descriptor. If poll event splits initiated timeout
interval then the reminder is calculated and still waited in the
following evlist__poll() call.

Committer testing:

The testing instructions came in the cover letter, here I'll extract the
parts that are needed to test this specific patch, so that we don't
introduce bisection regressions by testing only the patch series as a
whole:

<FILL IN THE TEST INSTRUCTIONS>

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/3cb8a826-145f-81f4-fcb2-fa20045c6957@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>