perf evlist: Implement control command handling functions

perf evlist: Implement control command handling functions

Implement functions of initialization, finalization and processing of
control command messages coming from control file descriptors.

Allocate control file descriptor as descriptor at struct pollfd object
of evsel_list for atomic poll() operation.

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/62518ceb-1cc9-2aba-593b-55408d07c1bf@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>