MEMBEROF: Do not add the ghost attribute to self
    
    When a nested group with ghost users is added, its ghost attribute should
    propagate within the nested group structure much like the memberuid
    attribute. Unlike the memberuid attribute, the ghost attribute is only
    semi-managed by the memberof plugin and added manually to the original
    entry.
    
    This bug caused LDB errors saying that attribute or value already exists
    when a group with a ghost user was added to the hierarchy as groups were
    updated with an attribute they already had.