Refactor ALPN/NPN protocol selection and introduce NGHTTP2_PROTO_ALPN macro

lib/includes/nghttp2/nghttp2.h

@@ -49,6 +49,24 @@ extern "C" {
  */
 #define NGHTTP2_PROTO_VERSION_ID_LEN 5
 
+/**
+ * @macro
+ *
+ * The seriazlied form of ALPN protocol identifier this library
+ * supports.  Notice that first byte is the length of following
+ * protocol identifier.  This is the same wire format of `TLS ALPN
+ * extension <https://tools.ietf.org/html/rfc7301>`_.  This is useful
+ * to process incoming ALPN tokens in wire format.
+ */
+#define NGHTTP2_PROTO_ALPN "\x5h2-14"
+
+/**
+ * @macro
+ *
+ * The length of :macro:`NGHTTP2_PROTO_ALPN`.
+ */
+#define NGHTTP2_PROTO_ALPN_LEN (sizeof(NGHTTP2_PROTO_ALPN) - 1)
+
 /**
  * @macro
  *