replace rot13 with tr implementation, since not everyone has /usr/bin/rot13
fix #10342: make circuit an attribute of transport, rather than passing it in as method params
there is actually only ever one circuit per transport instance, see #10342
add a transport_name field to BaseTransport, SOCKSv4Factory and StaticDestinationServerFactory
- later this will allow ezpt to handle differently-named transports
add a simple "id" transport that does no transformation
register the different ezpt transports as separate transports in their own right