New ot API to set a message to use direct tx (#1245)

- New OpenThread API `otMessageSetDirectTransmission()` to force
  a message to forwarded using direct transmission even if the
  destination is a sleepy-node.

- The new API is used from `NcpBase` from set handler of insecure
  network stream spinel `STREAM_NET_INSECURE` property.

include/openthread-message.h

@@ -177,6 +177,17 @@ ThreadError otSetMessageOffset(otMessage aMessage, uint16_t aOffset);
  */
 bool otIsMessageLinkSecurityEnabled(otMessage aMessage);
 
+/**
+ * This function sets/forces the message to be forwarded using direct transmission.
+ * Default setting for a new message is `false`.
+ *
+ * @param[in]  aMessage  A pointer to a message buffer.
+ * @param[in]  aEnabled  If `true` message will be forced to use direct transmission. If `false` message will
+ *                       follow the normal procedure.
+ *
+ */
+void otMessageSetDirectTransmission(otMessage aMessage, bool aEnabled);
+
 /**
  * Append bytes to a message.
  *