Add initial support for upgrading db4 DBs built with older DB4 versions

  DB4 data base files are tied to the version of DB4 that created them.
  It'd be nice to be able to compile with a more recent version of db4
  than db4.2, which is currently used for the Debian packages. However
  we don't want to have dump and reload the database, so this adds a
  first cut at trying to use the DB4 provided upgrade functions.