queue: Send delivery status notifications on failures

When we permanently failed to deliver to one or more recipients, send delivery
status notifications back to the sender.

To do this, we need to extend a couple of internal structures, to keep track
of the original destinations (so we can include them in the message, for
reference), and the hostname we're identifying ourselves as (this is arguable
but we're going with it for now, may change later).

internal/queue/dsn_test.go

@@ -0,0 +1,27 @@
+package queue
+
+import "testing"
+
+func TestDSN(t *testing.T) {
+	item := &Item{
+		Message: Message{
+			ID:   <-newID,
+			From: "from@from.org",
+			To:   []string{"toto@africa.org", "negra@sosa.org"},
+			Rcpt: []*Recipient{
+				{"poe@rcpt", Recipient_EMAIL, Recipient_FAILED,
+					"oh! horror!"},
+				{"newman@rcpt", Recipient_EMAIL, Recipient_FAILED,
+					"oh! the humanity!"}},
+			Data:     []byte("data ñaca"),
+			Hostname: "from.org",
+		},
+	}
+
+	msg, err := deliveryStatusNotification(item)
+	if err != nil {
+		t.Error(err)
+	}
+
+	t.Log(string(msg))
+}