cmd/tailscale,ipn: add auto-update flags and prefs (#8861)

The flags are hidden for now. Adding propagation to tailscaled and
persistence only. The prefs field is wrapped in a struct to allow for
future expansion (like update schedule).

Updates #6907

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -127,9 +127,10 @@ func (b *LocalBackend) handleC2NUpdate(w http.ResponseWriter, r *http.Request) {
 	//
 	// Note that we create the Updater solely to check for errors; we do not
 	// invoke it here. For this purpose, it is ok to pass it a zero Arguments.
+	prefs := b.Prefs().AutoUpdate()
 	_, err := clientupdate.NewUpdater(clientupdate.Arguments{})
 	res := tailcfg.C2NUpdateResponse{
-		Enabled:   envknob.AllowsRemoteUpdate(),
+		Enabled:   envknob.AllowsRemoteUpdate() || prefs.Apply,
 		Supported: err == nil && !version.IsMacSysExt(),
 	}