This change adds a `port_mapping` option to the `ReverseTunnel` definition in the (YAML) restriction file.
It maps ports on the server side from X to Y (X:Y). Where X is the originally requested port by the client and Y is the port which will be used to listen on server-side.
For example with `10001:8080` configured and a client which connects using `-R tcp://10001:localhost:80` the server will listen on port 8080 instead of 10001. The originally requested ports (NOT the mapped ports) still needs to be allowed via the `ports` directive.
This is for example useful when dealing with lots of clients and you don't want to coordinate port use on all the clients but centrally on the server.
