queue: Use a local envelope-from when forwarding

If there's an alias to forward email to a non-local domain, using the original
From is problematic, as we may not be an authorized sender for it.

Some MTAs (like Exim) will do it anyway, others (like gmail) will construct a
special address based on the original address.

This patch implements the latter approach, which is safer and allows the
receiver to properly enforce SPF.

We construct a (hopefully) reasonable From based on the local user, and
embedding the original From (but transformed for IDNA, as the receiver may not
support SMTPUTF8).

internal/queue/dsn_test.go

@@ -10,9 +10,9 @@ func TestDSN(t *testing.T) {
 			To:   []string{"toto@africa.org", "negra@sosa.org"},
 			Rcpt: []*Recipient{
 				{"poe@rcpt", Recipient_EMAIL, Recipient_FAILED,
-					"oh! horror!"},
+					"oh! horror!", ""},
 				{"newman@rcpt", Recipient_EMAIL, Recipient_FAILED,
-					"oh! the humanity!"}},
+					"oh! the humanity!", ""}},
 			Data:     []byte("data ñaca"),
 			Hostname: "from.org",
 		},