sexpots: Add extra debugging for displaying send/received characters on the serial line

I'm having some problems with connections getting drops under some odd circumstances, for example, when pressing # to connect to the viewdata service using BeebEm with a real modem attached to a windows machine. Connecting to the terminal works fine.

Being able to see characters received/sent on the serial line may be able to give me a clue as to what is being sent that I might not be picking up. It may also help in other situations.