Series3a trouble

There's a bug in the firmware of all Series 3a machines I've ever tried.

When sending a reply to a message on a channel, instead of sending the
reply to the channel that sent the message they send the reply down the
channel which was last opened.

So when receiving Series 3 packets, redirect the channel on the received
message to the channel from which a message was last sent. I'm pretty sure
Series 3 devices don't overlap messages in practice.

This patch gets plpftp working for me. Without it, plpftp hangs retrieving
owner info. It used to work, because the querying channel was the last
opened. I guess the clip server channel put paid to that.