lib/connections: Handle QUIC not being available (#7186)

This does two things:

- Exclude QUIC from go1.16 builds, automatically, for now, since it
  doesn't work and just panics.

- Provide some fake listeners and dialers when QUIC is disabled.

These fake listeners and dialers indicate that they are disabled and
unsupported, which silences "Dialing $address: unknown address scheme:
quic" type of stuff which is not super helpful to the user.

lib/connections/service.go

@@ -41,8 +41,15 @@ var (
 )
 
 var (
-	errDisabled   = errors.New("disabled by configuration")
-	errDeprecated = errors.New("deprecated protocol")
+	// Dialers and listeners return errUnsupported (or a wrapped variant)
+	// when they are intentionally out of service due to configuration,
+	// build, etc. This is not logged loudly.
+	errUnsupported = errors.New("unsupported protocol")
+
+	// These are specific explanations for errUnsupported.
+	errDisabled   = fmt.Errorf("%w: disabled by configuration", errUnsupported)
+	errDeprecated = fmt.Errorf("%w: deprecated", errUnsupported)
+	errNotInBuild = fmt.Errorf("%w: disabled at build time", errUnsupported)
 )
 
 const (
@@ -336,7 +343,6 @@ func (s *service) handle(ctx context.Context) error {
 		s.model.AddConnection(modelConn, hello)
 		continue
 	}
-	return nil
 }
 
 func (s *service) connect(ctx context.Context) error {
@@ -441,16 +447,10 @@ func (s *service) connect(ctx context.Context) error {
 				if err != nil {
 					s.setConnectionStatus(addr, err)
 				}
-				switch err {
-				case nil:
-					// all good
-				case errDisabled:
-					l.Debugln("Dialer for", uri, "is disabled")
+				if errors.Is(err, errUnsupported) {
+					l.Debugf("Dialer for %v: %v", uri, err)
 					continue
-				case errDeprecated:
-					l.Debugln("Dialer for", uri, "is deprecated")
-					continue
-				default:
+				} else if err != nil {
 					l.Infof("Dialer for %v: %v", uri, err)
 					continue
 				}
@@ -505,7 +505,6 @@ func (s *service) connect(ctx context.Context) error {
 			return ctx.Err()
 		}
 	}
-	return nil
 }
 
 func (s *service) isLANHost(host string) bool {
@@ -634,16 +633,10 @@ func (s *service) CommitConfiguration(from, to config.Configuration) bool {
 		}
 
 		factory, err := getListenerFactory(to, uri)
-		switch err {
-		case nil:
-			// all good
-		case errDisabled:
-			l.Debugln("Listener for", uri, "is disabled")
+		if errors.Is(err, errUnsupported) {
+			l.Debugf("Listener for %v: %v", uri, err)
 			continue
-		case errDeprecated:
-			l.Debugln("Listener for", uri, "is deprecated")
-			continue
-		default:
+		} else if err != nil {
 			l.Infof("Listener for %v: %v", uri, err)
 			continue
 		}