tools lib: Update copy of strtobool from the kernel sources

Getting support for "on", "off" introduced in a81a5a17 ("lib: add
"on"/"off" support to kstrtobool") and making it check for NULL,
introduced in ef951599

 ("lib: move strtobool() to kstrtobool()").

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>
Cc: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/n/tip-mu8ghin4rklacmmubzwv8td7@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>