db_tools::create_unique_index does not use specified index names on MySQL

The generated code should be ALTER TABLE tablename ADD UNIQUE INDEX indexname (columns);

Currently the indexname is not used, leading to the first column name being used as default which can cause problems if you later want to drop an INDEX by name in the updater.

We will have to do some analysis to see if we have added unique indexes in updates before and whether we can identify and rename them safely.