(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>
Sync with gnulib 94ee038129c3b40510ef0ff6ac6ddfe85bf0f97f.

(xstrtoumax): use enum instead of 0, to pacify IRIX 6.5 cc.

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