Added NSAPI_KEEPINTVL/NSAPI_KEEPIDLE options to the socket API

Author: geky

NetworkStack.h

@@ -73,6 +73,8 @@ enum nsapi_level_t {
 enum nsapi_option_t {
     NSAPI_REUSEADDR, /*!< Allow bind to reuse local addresses */
     NSAPI_KEEPALIVE, /*!< Enables sending of keepalive messages */
+    NSAPI_KEEPIDLE,  /*!< Sets timeout value to initiate keepalive */
+    NSAPI_KEEPINTVL, /*!< Sets timeout value for keepalive */
     NSAPI_LINGER,    /*!< Keeps close from returning until queues empty */
     NSAPI_SNDBUF,    /*!< Sets send buffer size */
     NSAPI_RCVBUF,    /*!< Sets recv buffer size */