cmd/*, lib/tlsutil: Refactor TLS stuff (fixes #5256) (#5276)

This changes the TLS and certificate handling in a few ways:

- We always use TLS 1.2, both for sync connections (as previously) and
  the GUI/REST/discovery stuff. This is a tightening of the requirements
  on the GUI. AS far as I can tell from caniusethis.com every browser from
  2013 and forward supports TLS 1.2, so I think we should be fine.

- We always greate ECDSA certificates. Previously we'd create
  ECDSA-with-RSA certificates for sync connections and pure RSA
  certificates for the web stuff. The new default is more modern and the
  same everywhere. These certificates are OK in TLS 1.2.

- We use the Go CPU detection stuff to choose the cipher suites to use,
  indirectly. The TLS package uses CPU capabilities probing to select
  either AES-GCM (fast if we have AES-NI) or ChaCha20 (faster if we
  don't). These CPU detection things aren't exported though, so the tlsutil
  package now does a quick TLS handshake with itself as part of init().
  If the chosen cipher suite was AES-GCM we prioritize that, otherwise we
  prefer ChaCha20. Some might call this ugly. I think it's awesome.

cmd/syncthing/gui.go

@@ -185,28 +185,13 @@ func (s *apiService) getListener(guiCfg config.GUIConfiguration) (net.Listener,
 			name = tlsDefaultCommonName
 		}
 
-		cert, err = tlsutil.NewCertificate(s.httpsCertFile, s.httpsKeyFile, name, httpsRSABits)
+		cert, err = tlsutil.NewCertificate(s.httpsCertFile, s.httpsKeyFile, name)
 	}
 	if err != nil {
 		return nil, err
 	}
-	tlsCfg := &tls.Config{
-		Certificates: []tls.Certificate{cert},
-		MinVersion:   tls.VersionTLS10, // No SSLv3
-		CipherSuites: []uint16{
-			// No RC4
-			tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
-			tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
-			tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
-			tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
-			tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
-			tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA,
-			tls.TLS_RSA_WITH_AES_128_CBC_SHA,
-			tls.TLS_RSA_WITH_AES_256_CBC_SHA,
-			tls.TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA,
-			tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA,
-		},
-	}
+	tlsCfg := tlsutil.SecureDefault()
+	tlsCfg.Certificates = []tls.Certificate{cert}
 
 	if guiCfg.Network() == "unix" {
 		// When listening on a UNIX socket we should unlink before bind,