Add "server reachability" register to SNTP

Guarded with SNTP_MONITOR_SERVER_REACHABILITY (sntp_opts.h).
See patch 9581

Signed-off-by: goldsimon <goldsimon@gmx.de>

src/include/lwip/apps/sntp.h

@@ -58,6 +58,10 @@ u8_t sntp_enabled(void);
 void sntp_setserver(u8_t idx, const ip_addr_t *addr);
 const ip_addr_t* sntp_getserver(u8_t idx);
 
+#if SNTP_MONITOR_SERVER_REACHABILITY
+u8_t sntp_getreachability(u8_t idx);
+#endif /* SNTP_MONITOR_SERVER_REACHABILITY */
+
 #if SNTP_SERVER_DNS
 void sntp_setservername(u8_t idx, const char *server);
 const char *sntp_getservername(u8_t idx);

src/include/lwip/apps/sntp_opts.h

@@ -195,6 +195,13 @@
 #define SNTP_RETRY_TIMEOUT_EXP      1
 #endif
 
+/** Keep a reachability shift register per server
+ * Default is on to conform to RFC.
+ */
+#if !defined SNTP_MONITOR_SERVER_REACHABILITY || defined __DOXYGEN__
+#define SNTP_MONITOR_SERVER_REACHABILITY 1
+#endif
+
 /**
  * @}
  */