listed()
  SQL blacklist for a policy.
  
  You can use this scanner to chcek, if an record is present in SQL table.
  
  Usage: listed(flags='B', table='greylist')
  
  Where: flags is an character, which defines which flag will be searched
           in SQL table. By default 'B' for this scanner.
         table can define table alternative
  
  Returns: level=1.0, when client is in blacklist
           level=0.0, when client is not in blacklist
  
  Example: listed()
  
  New in version 0.8.0.