Resolves: #231093
    Summary: db2bak: crash bug (comment #8, 11)
    Description:
    Set the strong requirement: nsslapd-directory must have some value.
    to guarantee it:
    1) checking errors from ldbm_config_directory_set. If the check fails, don't
    start the server.
    2) if nsslapd-directory does not exist or the value is empty in dse.ldif,
    issuing an error message and returning the error code.
    3) since it was difficult to distinguish the nsslapd-directory empty value from
    the initial default value, introduced CONFIG_FLAG_SKIP_DEFAULT_SETTING flag to
    tell the backend config code to skip setting the default value