(mc_config_get_keys): optimization of function itself and usage.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(mc_config_get_groups): optimization of function itself and usage.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(mc_config_get_string_list): clarify usage.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>