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/ipn_clone.go

@@ -51,6 +51,7 @@ var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	NetfilterMode          preftype.NetfilterMode
 	OperatorUser           string
 	ProfileName            string
+	AutoUpdate             AutoUpdatePrefs
 	Persist                *persist.Persist
 }{})