Slight change in procedure: a 'master' database must now be specified. The...

Slight change in procedure: a 'master' database must now be specified. The external_dbs.txt file is loaded into this, and this in turn is compared (id and name) with the contents of the external_db table in each of the databases to be updated.

If the database to be updated has any extra rows, a warning is given and the database is skipped.

This should help avoid unilateral changes to external_db which haven't been commited to the external_dbs.txt file in CVS being silently overwritten and causing problems down the line.