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>

src/bacnet/basic/object/bo.c

@@ -1154,7 +1154,7 @@ bool Binary_Output_Write_Property(BACNET_WRITE_PROPERTY_DATA *wp_data)
             break;
     }
     /* not using len at this time */
-    len = len;
+    (void)len;
 
     return status;
 }