Enhancement: print actual listen address after start-up

When started with `--listen :0` the server would print `start server on :0`

Now the message includes the actual address listened on, for example
`start server on 0.0.0.0:37333`.

This is useful when starting a server with an auto-allocated free port number (port 0).

https://github.com/restic/rest-server/pull/271