[proxy] Handle some unix signals.

On the surface, this doesn't add much, but there are some benefits:

* We can do graceful shutdowns and thus record more code coverage data.

* We now have a foundation for the more interesting behaviors, e.g. "stop
  accepting new connections after SIGTERM but keep serving the existing ones".

* We give the otel machinery a chance to flush trace events before
  finally shutting down.

proxy/src/http/websocket.rs

@@ -186,8 +186,8 @@ async fn ws_handler(
 }
 
 pub async fn task_main(
-    ws_listener: TcpListener,
     config: &'static ProxyConfig,
+    ws_listener: TcpListener,
 ) -> anyhow::Result<()> {
     scopeguard::defer! {
         info!("websocket server has shut down");