Bug 434735 - Allow SASL ANONYMOUS mech to work
    
    The SASL ANONYMOUS mechanism was broken since the SASL mapping code
    was invoked to map anonymous to a real user entry.  This adds a
    special case to the canonify user callback that sets the bind DN
    to "" if the ANONYMOUS mechanism is being used.
    
    I also added a check to see if anonymous access is disabled when
    we set up the SASL secprops for a connection.  If anonymous access
    is disabled, we set a SASL security flag to disallow mechanisms
    that would allow anonymous access.