Bug 663484 - Entry usn plugin fails to properly tag entries on initialization
    
    https://bugzilla.redhat.com/show_bug.cgi?id=663484
    
    Description:
    1) Introducing a config parameter nsslapd-entryusn-import-initval.
    If the value is digit, e.g., 0, 10, 100 ..., the imported entries
    will have the entryusn value.
    If the value is not digit, e.g., "next", the imported entries
    will have the last entryusn + 1 from the database existed before
    the import was exected or initialized by the master.
    
    2) Internal OID LDBM_ENTRYUSN_OID has been redifined in back-ldbm.h
    to avoid a possible conflict.
    
    3) If an attribute to modify is an operational attribute and
    that is the only one modify operation (not one of the series
    of modify operations), the server returns UNWILLING_TO_PERFORM.
    
    In addition, this patch fixes a minor invalid access to a freed
    memory in cl5DeleteDBSync.