Enhancement: after start-up, server served message now prints actual listen address

For example, when started with `--listen :0`, previously the server would print:

```
start server on :0
```

Now it will print the actual address listened on, e.g.

```
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