fix verifying dane-ta connections for outgoing email where the dane-t…

…a record is not for the first certificate in the chain after the leaf certifiate.

tls servers send a list of certificates for the connection. the first is the
leaf certificate. that's the one for the server itself. that's the one we want
to verify. the others are intermediate CA's. and possibly even the root CA
certificate that it hopes is trusted at the client (though sending it doesn't
make it trusted). with dane-ta, the public key of an intermediate or root CA
certificate is listed in the TSLA record. when verifying, we add any
intermediate/root CA that matches a dane-ta tlsa record to the trusted root CA
certs. we should also have added CA certs that didn't match a TLSA record to
the "intermediates" of x509.VerifyOptions. because we didn't,
x509.Certificate.Verify couldn't verify the chain from the trusted dane-ta ca
cert to the leaf cert. we would only properly verify a dane-ta connection
correctly if the dane-ta-trusted ca cert was the one immediately following the
leaf cert. not when there were one or more additional intermediate certs.

this showed when connecting to mx.runbox.com.

problem reported by robbo5000 on matrix, thanks!

dane/dane.go

@@ -448,7 +448,8 @@ func verifySingle(log mlog.Log, tlsa adns.TLSA, cs tls.ConnectionState, allowedH
 		// We set roots, so the system defaults don't get used. Verify checks the host name
 		// (set below) and checks for expiration.
 		opts := x509.VerifyOptions{
-			Roots: x509.NewCertPool(),
+			Intermediates: x509.NewCertPool(),
+			Roots:         x509.NewCertPool(),
 		}
 
 		// If the full certificate was included, we must add it to the valid roots, the TLS
@@ -465,11 +466,13 @@ func verifySingle(log mlog.Log, tlsa adns.TLSA, cs tls.ConnectionState, allowedH
 			}
 		}
 
-		for _, cert := range cs.PeerCertificates {
+		for i, cert := range cs.PeerCertificates {
 			if match(cert) {
 				opts.Roots.AddCert(cert)
 				found = true
 				break
+			} else if i > 0 {
+				opts.Intermediates.AddCert(cert)
 			}
 		}
 		if !found {