libsubcmd: Introduce OPT_CALLBACK_SET()

To register that an option was set, like with the upcoming 'perf record
--switch-output-option' one.

Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: http://lore.kernel.org/lkml/20200429131106.27974-7-acme@kernel.org


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