Allow the data responses to send output to the main connection.

[samdeane]

There's currently no way for the main connection to know when the data responses have been sent down the data listener's connection.

This makes it impossible to correctly synchronise "transfer complete" type messages down the main connection - instead we just have to add a pause before sending them, and hope that the data transfer is done in time.

We should add a command that can be executed as part of the data connection's initial responses commands, which generates output to be sent to the main connection. This will allow the responses to be sent at the correct time.