INI: Add INI_MS_DETECT merge notifications
    
    In ini_config_augment, INI_MS_DETECT is supposed to detect
    duplicate sections. Previously this was exposed only as
    a return status of EEXIST. This updates the behavior to
    return an error_list with warnings of files containing
    one or more duplicate sections.
    
    Signed-off-by: Alexander Scheel <ascheel@redhat.com>
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    
    Merges: https://pagure.io/SSSD/ding-libs/issue/3167