(Clone Detection) 31-func-similarity-worklist can exclude functions

* 31-func-similarity-worklist has an --exclude-functions switch that
  takes the name of a DB table (and column) that lists function IDs
  that should not be part of the generated worklist.

* The io_uninteresting.sql script creates an io_uninteresting table
  that contains functions whose I/O behavior is uninteresting
  according to the criteria in that script.