ipn/ipnlocal: periodically run auto-updates when "offline" (#12118)

When the client is disconnected from control for any reason (typically
just turned off), we should still attempt to update if auto-updates are
enabled. This may help users who turn tailscale on infrequently for
accessing resources.

RELNOTE: Apply auto-updates even if the node is down or disconnected
from the coordination server.

Updates #12117

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

ipn/ipnlocal/autoupdate_disabled.go

@@ -0,0 +1,18 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !(linux || windows)
+
+package ipnlocal
+
+import (
+	"tailscale.com/ipn"
+)
+
+func (b *LocalBackend) stopOfflineAutoUpdate() {
+	// Not supported on this platform.
+}
+
+func (b *LocalBackend) maybeStartOfflineAutoUpdate(prefs ipn.PrefsView) {
+	// Not supported on this platform.
+}