Repair broken mysqldump

#925776: Refactor _drush_sql_get_credentials() to allow empty passwords broke sql-sync. This patch fixes it. --databases was wrong because it writes the database name into the dump file, making the dump file useless for anything other than restoring to the same database name. sql-sync often needs to copy from one database to another, often with databases with different names.

Already committed; too serious to leave unfixed. Please re-open if necessary, but in short, sql-sync does not work without this patch and does work with it.