INI: Prepare for schema validation
    
    Pointer to function ini_schema_validator_func will be an optional
    and can be used to schema validation. It shoul also prepare data
    for validator function if last output argument is not NULL.
    These prepared data will be passed to validator function as
    a 5th argument of ini_validator_func. These two functions
    are responsible for memory management of passed additional data.
    
    It isn't an API/ABI change because lib_iniconfig has not been released yet.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>