Implemented basic icy support for the httpd output

[mk: folded with patch "Put icy related functions in extra source
files"; moved icy_server.c from HAVE_CURL to ENABLE_HTTPD_OUTPUT;
removed an unused variable]

src/output/httpd_internal.h

@@ -81,6 +81,11 @@ struct httpd_output {
 	 */
 	struct page *header;
 
+	/**
+	 * The metadata, which is sent to every client.
+	 */
+	struct page *metadata;
+
 	/**
 	 * A linked list containing all clients which are currently
 	 * connected.