config/Param: split block-specific attributes to new struct ConfigBlock

The old struct config_param remains only for top-level string options.

test/run_output.cxx

@@ -61,8 +61,8 @@ PlayerControl::~PlayerControl() {}
 static AudioOutput *
 load_audio_output(EventLoop &event_loop, const char *name)
 {
-	const config_param *param =
-		config_find_block(ConfigOption::AUDIO_OUTPUT, "name", name);
+	const auto *param = config_find_block(ConfigBlockOption::AUDIO_OUTPUT,
+					      "name", name);
 	if (param == NULL) {
 		fprintf(stderr, "No such configured audio output: %s\n", name);
 		return nullptr;