Ticket #48178 add config param to enable nunc-stans
    
    https://fedorahosted.org/389/ticket/48178
    Reviewed by: nhosoi (Thanks!)
    Branch: master
    Fix Description: If nsslapd-enable-nunc-stans: on, the server will use
    nunc-stans as the event framework.  NOTE: You still have to build the
    server with configure --enable-nunc-stans in order to be able to set
    nsslapd-enable-nunc-stans: on.  By default, the value is off.
    Platforms tested: Fedora 20
    Flag Day: no
    Doc impact: no