Set config_name as a unique index

Work around possible duplicates by keeping the first record.

sql-schema/109.sql

@@ -0,0 +1,5 @@
+CREATE TABLE tmp_table LIKE config;
+ALTER IGNORE TABLE tmp_table ADD UNIQUE INDEX uniqueindex_configname (config_name);
+INSERT IGNORE INTO tmp_table SELECT * FROM config;
+DROP TABLE config;
+RENAME TABLE tmp_table TO config;