Ticket 49231 - fix sasl mech handling
    
    Bug Description:  In our sasl code we had two issues. One was that
    we did not correctly apply the list of sasl allowed mechs to our
    rootdse list in ids_sasl_listmech. The second was that on config
    reset, we did not correctly set null to the value.
    
    Fix Description:  Fix the handling of the mech lists to allow
    reset, and allow the mech list to be updated properly.
    
    https://pagure.io/389-ds-base/issue/49231
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)
    
    (cherry picked from commit 639535771d13de86446a361a515713ae15f65722)