rc: In options/info make FieldName contain a "." if it should be nested

Before this would have Output "FieldName": "ListenAddr" where it
actually needs to be set in a sub object "HTTP".

After this fix it outputs "FieldName": "HTTP.ListenAddr" to indicate
"ListenAddr" needs to be set in the object "HTTP".

fs/config/configstruct/configstruct.go

@@ -139,6 +139,7 @@ func Items(opt any) (items []Item, err error) {
 				if hasTag {
 					newItem.Name = configName + "_" + newItem.Name
 				}
+				newItem.Field = fieldName + "." + newItem.Field
 				items = append(items, newItem)
 			}
 		} else {