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.

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

ports/bsd/bacport.h

@@ -88,10 +88,12 @@
 #include <sys/un.h>
 #include <sys/ioctl.h>
 #include <netdb.h>
+#include "bacnet/bacnet_stack_exports.h"
 
 /** @file bsd/net.h  Includes BSD network headers. */
 
 /* Local helper functions for this port */
+BACNET_STACK_EXPORT
 extern int bip_get_local_netmask(
     struct in_addr *netmask);