Add bip_get_socket (#210)

* Add function to get BIP socket

* Add bip_get_socket to all applicable ports

ports/linux/bip-init.c

@@ -94,6 +94,17 @@ static void debug_print_ipv4(const char *str,
     }
 }
 
+
+/**
+ * @brief Return the active BIP socket.
+ * @return The active BIP socket, or -1 if uninitialized.
+ */
+int bip_get_socket(void)
+{
+    return BIP_Socket;
+}
+
+
 /**
  * @brief Enabled debug printing of BACnet/IPv4
  */