feat: print signal causing shutdown

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

app.go

@@ -53,7 +53,8 @@ func serve(sig chan os.Signal, db *leveldb.DB) error {
 
 	// graceful shutdown
 	go func() {
-		<-sig
+		s := <-sig
+		log.Printf("received %s", s)
 		log.Println("shutting down")
 		if err := app.Shutdown(); err != nil {
 			fmt.Printf("cannot shutdown: %v", err)