move ISUPPORT regeneration to the end of config loading

irc/config.go

 
 	config.loadMOTD()
 
-	err = config.generateISupport()
-	if err != nil {
-		return nil, err
-	}
-
 	// in the current implementation, we disable history by creating a history buffer
 	// with zero capacity. but the `enabled` config option MUST be respected regardless
 	// of this detail
 		}
 	}
 
+	// now that all postprocessing is complete, regenerate ISUPPORT:
+	err = config.generateISupport()
+	if err != nil {
+		return nil, err
+	}
+
 	return config, nil
 }