Print message when stopping the daemon (SIGTERM)

systemd sends SIGTERM when stopping a service. When running in a tty,
CTRL + C (SIGINT) currently won't print a shutdown message.

sfsmcd/src/main.rs

@@ -55,6 +55,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     // Will (afaik) be stabilized in Rust 1.48
     // core::future::pending::<()>().await;
-    loop {};
-    unreachable!();
+    tokio::signal::unix::signal(tokio::signal::unix::SignalKind::terminate())?.recv().await;
+    info!("Got SIGTERM, shutting down");
+    Ok(())
 }