courier: Add STS policy checking to the SMTP courier

This patch extends the SMTP courier to (optionally) do STS policy
checking when delivering mail.

As STS support is currently experimental, we gate this behind a flag and
is disabled by default.

internal/courier/smtp_test.go

@@ -23,7 +23,7 @@ func newSMTP(t *testing.T) (*SMTP, string) {
 		t.Fatal(err)
 	}
 
-	return &SMTP{dinfo}, dir
+	return &SMTP{dinfo, nil}, dir
 }
 
 // Fake server, to test SMTP out.