support udp socket connect (#1526)

include/openthread/udp.h

@@ -99,6 +99,7 @@ otMessage *otUdpNewMessage(otInstance *aInstance, bool aLinkSecurityEnabled);
  * @sa otUdpNewMessage
  * @sa otUdpClose
  * @sa otUdpBind
+ * @sa otUdpConnect
  * @sa otUdpSend
  *
  */
@@ -114,6 +115,7 @@ ThreadError otUdpOpen(otInstance *aInstance, otUdpSocket *aSocket, otUdpReceive
  * @sa otUdpNewMessage
  * @sa otUdpOpen
  * @sa otUdpBind
+ * @sa otUdpConnect
  * @sa otUdpSend
  *
  */
@@ -129,12 +131,30 @@ ThreadError otUdpClose(otUdpSocket *aSocket);
  *
  * @sa otUdpNewMessage
  * @sa otUdpOpen
+ * @sa otUdpConnect
  * @sa otUdpClose
  * @sa otUdpSend
  *
  */
 ThreadError otUdpBind(otUdpSocket *aSocket, otSockAddr *aSockName);
 
+/**
+ * Connect a UDP/IPv6 socket.
+ *
+ * @param[in]  aSocket    A pointer to a UDP socket structure.
+ * @param[in]  aSockName  A pointer to an IPv6 socket address structure.
+ *
+ * @retval kThreadErrorNone  Connect operation was successful.
+ *
+ * @sa otUdpNewMessage
+ * @sa otUdpOpen
+ * @sa otUdpBind
+ * @sa otUdpClose
+ * @sa otUdpSend
+ *
+ */
+ThreadError otUdpConnect(otUdpSocket *aSocket, otSockAddr *aSockName);
+
 /**
  * Send a UDP/IPv6 message.
  *
@@ -146,6 +166,7 @@ ThreadError otUdpBind(otUdpSocket *aSocket, otSockAddr *aSockName);
  * @sa otUdpOpen
  * @sa otUdpClose
  * @sa otUdpBind
+ * @sa otUdpConnect
  * @sa otUdpSend
  *
  */