cmd/tailscaled, tailcfg, hostinfo: add flag to disable logging + support

As noted in #5617, our documented method of blocking log.tailscale.io
DNS no longer works due to bootstrap DNS.

Instead, provide an explicit flag (--no-logs-no-support) and/or env
variable (TS_NO_LOGS_NO_SUPPORT=true) to explicitly disable logcatcher
uploads. It also sets a bit on Hostinfo to say that the node is in that
mode so we can end any support tickets from such nodes more quickly.

This does not yet provide an easy mechanism for users on some
platforms (such as Windows, macOS, Synology) to set flags/env. On
Linux you'd used /etc/default/tailscaled typically. Making it easier
to set flags for other platforms is tracked in #5114.

Fixes #5617
Fixes tailscale/corp#1475

Change-Id: I72404e1789f9e56ec47f9b7021b44c025f7a373a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logpolicy/logpolicy.go

@@ -539,7 +539,10 @@ func New(collection string) *Policy {
 		conf.IncludeProcSequence = true
 	}
 
-	if val := getLogTarget(); val != "" {
+	if envknob.NoLogsNoSupport() {
+		log.Println("You have disabled logging. Tailscale will not be able to provide support.")
+		conf.HTTPC = &http.Client{Transport: noopPretendSuccessTransport{}}
+	} else if val := getLogTarget(); val != "" {
 		log.Println("You have enabled a non-default log target. Doing without being told to by Tailscale staff or your network administrator will make getting support difficult.")
 		conf.BaseURL = val
 		u, _ := url.Parse(val)
@@ -735,3 +738,14 @@ func goVersion() string {
 	}
 	return v
 }
+
+type noopPretendSuccessTransport struct{}
+
+func (noopPretendSuccessTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	io.ReadAll(req.Body)
+	req.Body.Close()
+	return &http.Response{
+		StatusCode: 200,
+		Status:     "200 OK",
+	}, nil
+}