fix BACnet/IP on OS to bind broadcast to specific port (#489)

* Fixes #346 by binding the broadcast socket to the port specific broadcast address rather than INADDR_ANY or INADDR_BROADCAST. Added additional compile and environment options for those who might need them. 

* Changed the define USE_CLASSADDR to BACNET_IP_BROADCAST_USE_CLASSADDR to help manage namespaces.

* Added BACNET_IP_BROADCAST_BIND_ADDR environment variable to override the default broadcast binding address.

---------

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

ports/win32/bacport.h

@@ -39,7 +39,7 @@
 #endif
 
 #include <windows.h>
-#if (!defined(USE_CLASSADDR) || (USE_CLASSADDR == 0))
+#ifndef BACNET_IP_BROADCAST_USE_CLASSADDR
 #include <iphlpapi.h>
 #if defined(_MSC_VER)
 #pragma comment(lib, "IPHLPAPI.lib")
@@ -81,6 +81,10 @@ and globals in favor of more secure versions.  */
 #define snprintf _snprintf
 #endif
 
+BACNET_STACK_EXPORT
+extern int bip_get_local_netmask(
+    struct in_addr *netmask);
+
 #define BACNET_OBJECT_TABLE(table_name, _type, _init, _count,               \
                             _index_to_instance, _valid_instance, _object_name, \
                             _read_property, _write_property, _RPM_list,     \