perf sort: Provide a way to find out if per-thread bucketing is in place

Now the UI browsers will be able to offer thread related operations only
if the thread is part of the sort order in use, i.e. if hist_entry stats
are all for a single thread.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>,
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452960197-5323-9-git-send-email-namhyung@kernel.org


[ Carved out from a  larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>