control/controlhttp: don't require valid TLS cert for Noise connection

We don't require any cert at all for Noise-over-plaintext-port-80-HTTP,
so why require a valid cert chain for Noise-over-HTTPS? The reason we use
HTTPS at all is to get through firewalls that allow tcp/443 but not tcp/80,
not because we need the security properties of TLS.

Updates #3198

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlhttp/constants.go

@@ -78,9 +78,9 @@ type Dialer struct {
 	proxyFunc func(*http.Request) (*url.URL, error) // or nil
 
 	// For tests only
-	drainFinished     chan struct{}
-	insecureTLS       bool
-	testFallbackDelay time.Duration
+	drainFinished        chan struct{}
+	omitCertErrorLogging bool
+	testFallbackDelay    time.Duration
 }
 
 func strDef(v1, v2 string) string {