tools ui popup: Allow returning hotkeys

With this patch if an optional pointer is passed to ui__popup_menu()
then when any key that is not being handled (ENTER, ESC, etc) is typed,
it'll record that key in the pointer and return, allowing for hotkey
processing on the caller.

If NULL is passed, no change in logic, unhandled keys continue to be
ignored.

Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-6ojn19mqzgmrdm8kdoigic0m@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>