Ticket 48902 - Strdup pwdstoragescheme name to prevent misbehaving plugins
    
    Bug Description:  Some plugins would set the pwdstorageschemename to a value
    from their stack. This would cause ns-slapd to segfault on shutdown due to
    attempting to free this value.
    
    Fix Description:  pblock now strdups the pwdstorageschemename, so that plugins
    can behave however they want, and we always do the right thing.
    
    https://fedorahosted.org/389/ticket/48902
    
    Author: wibrown
    
    Review by: nhosoi (Thanks!)