perf script: Add --max-stack knob

Works just like with 'perf report'. In some cases we may want to have
more than 127 entries, the default maximum.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-mqkz2p5ok2978gztb0vsnocc@git.kernel.org


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