coap_io.h: define flag to indicate that data is available

Non-blocking sockets can set COAP_ENDPOINT_HAS_DATA in flags to tell
coap_read() that this endpoint has new data available for reading.

include/coap/coap_io.h

@@ -55,8 +55,10 @@ typedef struct coap_endpoint_t {
   int flags;
 } coap_endpoint_t;
 
-#define COAP_ENDPOINT_NOSEC 0x00
-#define COAP_ENDPOINT_DTLS  0x01
+#define COAP_ENDPOINT_NOSEC    0x0000
+#define COAP_ENDPOINT_DTLS     0x0001
+
+#define COAP_ENDPOINT_HAS_DATA 0x1000
 
 coap_endpoint_t *coap_new_endpoint(const coap_address_t *addr, int flags);