config-control syntax cleanup

Three issues:

• PARSER_CONFIG_CONTROL is not defined so there is a sub-parser for config-control but it is not usable.
• config_control is defined twice (bison behavior is not clear: it seems it uses only the second one). I removed the spurious first definition.
• unknown_map_entry is called with a short choice: when the parser finds something not matching a choice it emits a message with possible alternatives when their number is low, the unknown_map_entry handles cases where there are too many alternatives to get an suer friendly message. It does not make sense to use it with one alternative...