Ticket 48880 - adding pre/post extop ability
    
    Bug Description:
        In some cases it is useful to allow a plugin to change the target_dn before processing the extop.
        The extop does not support PRE/POST operation.
    
    Fix Description:
        extended operation would call SLAPI_PLUGIN_PRE_EXTOP_FN/SLAPI_PLUGIN_POST_EXTOP_FN callbacks.
        The plugins register those callbacks as SLAPI_PLUGIN_PRE_EXTOP_FN/SLAPI_PLUGIN_PRE_EXTOP_FN in their
        init function. The callbacks are then store into dedicated plugins lists SLAPI_PLUGIN_PREEXTOPERATION and
        SLAPI_PLUGIN_POSTEXTOPERATION
    
    http://fedorahosted.org/389/ticket/48880ticket URL>
    
    Reviewed by: William Brown (thanks)
    
    Platforms tested: F23
    
    Flag Day: no
    
    Doc impact: no