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

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap