tools lib subcmd: Add OPT_STRING_OPTARG_SET option

tools lib subcmd: Add OPT_STRING_OPTARG_SET option

To allow string options with a default argument and variable set when
the option is used.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Wang Nan <wangnan0@huawei.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1483431600-19887-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>