Require RTSP encryption when encryption mode is set to mandatory

This also lets us provide a friendly error to the client when it is rejected.

src/network.h

@@ -84,4 +84,12 @@ namespace net {
    */
   std::string
   addr_to_url_escaped_string(boost::asio::ip::address address);
+
+  /**
+   * @brief Returns the encryption mode for the given remote endpoint address.
+   * @param address The address used to look up the desired encryption mode.
+   * @return The WAN or LAN encryption mode, based on the provided address.
+   */
+  int
+  encryption_mode_for_address(boost::asio::ip::address address);
 }  // namespace net