Avoid duplication of fields in filter struct.

dnsforward/dnsforward.go

@@ -67,8 +67,8 @@ var defaultValues = ServerConfig{
 }
 
 type Filter struct {
-	ID    int64
-	Rules []string
+	ID    int64    `json:"id"`         // auto-assigned when filter is added (see nextFilterID), json by default keeps ID uppercase but we need lowercase
+	Rules []string `json:"-" yaml:"-"` // not in yaml or json
 }
 
 //