show transport activity when connecting

[Martin Pool]

For some commands the slowest part is opening an ssh connection to the server, and this is often also the source of problems if eg the machine can't be reached, the wrong machine is being contacted, or the user's account is not configured. It would be nice to systematically show a message while connecting so the user knows what's going on, perhaps as part of transport activity indicators.

At the moment it tends to show either nothing or "0KB 0KB/s" which looks a bit like it's hung.