fix: print the actual address listened on

This is useful when the server is started with an ephemeral port, e.g.:

./rest-server --no-auth --listen "127.0.0.1:0"

...

start server on 127.0.0.1:46015

changelog/unreleased/pull-271

@@ -0,0 +1,17 @@
+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

cmd/rest-server/listener_unix.go

@@ -28,7 +28,7 @@ func findListener(addr string) (listener net.Listener, err error) {
 			return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
 		}
 
-		log.Printf("start server on %v", addr)
+		log.Printf("start server on %v", listener.Addr())
 		return listener, nil
 
 	case 1:

cmd/rest-server/listener_windows.go

@@ -14,6 +14,6 @@ func findListener(addr string) (listener net.Listener, err error) {
 		return nil, fmt.Errorf("listen on %v failed: %w", addr, err)
 	}
 
-	log.Printf("start server on %v", addr)
+	log.Printf("start server on %v", listener.Addr())
 	return listener, nil
 }