Feature/what is network number handling (#304)

* Add What-Is-Network-Number handling.

Add What-Is-Network-Number and Network-Number-Is network layer handling.
Refactor npdu_encode_npdu_network() from router specific code.
Add unit test for NDPU network message
Add app for What-Is-Network-Number
Add app for Network-Number-Is
Add send helper for What-Is-Network-Number
Add send helper for Network-Number-Is

* added sys/debug.c to ports builds for use of debug_printf() in npdu handler.

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/npdu.h

@@ -61,6 +61,9 @@ typedef struct router_port_t {
     struct router_port_t *next;         /**< Point to next in linked list */
 } BACNET_ROUTER_PORT;
 
+#define NETWORK_NUMBER_LEARNED 0
+#define NETWORK_NUMBER_CONFIGURED 1
+
 #ifdef __cplusplus
 extern "C" {
 #endif /* __cplusplus */
@@ -83,6 +86,13 @@ extern "C" {
         bool data_expecting_reply,
         BACNET_MESSAGE_PRIORITY priority);
 
+    BACNET_STACK_EXPORT
+    void npdu_encode_npdu_network(
+        BACNET_NPDU_DATA *npdu_data,
+        BACNET_NETWORK_MESSAGE_TYPE network_message_type,
+        bool data_expecting_reply,
+        BACNET_MESSAGE_PRIORITY priority);
+
     BACNET_STACK_EXPORT
     void npdu_copy_data(
         BACNET_NPDU_DATA * dest,