cmd/{containerboot,k8s-operator},kube/kubetypes: unadvertise ingress services on shutdown (#15451)

Ensure no services are advertised as part of shutting down tailscaled.
Prefs are only edited if services are currently advertised, and they're
edited we wait for control's ~15s (+ buffer) delay to failover.

Note that editing prefs will trigger a synchronous write to the state
Secret, so it may fail to persist state if the ProxyGroup is getting
scaled down and therefore has its RBAC deleted at the same time, but that
failure doesn't stop prefs being updated within the local backend,
doesn't  affect connectivity to control, and the state Secret is
about to get deleted anyway, so the only negative side effect is a harmless
error log during shutdown. Control still learns that the node is no
longer advertising the service and triggers the failover.

Note that the first version of this used a PreStop lifecycle hook, but
that only supports GET methods and we need the shutdown to trigger side
effects (updating prefs) so it didn't seem appropriate to expose that
functionality on a GET endpoint that's accessible on the k8s network.

Updates tailscale/corp#24795

Change-Id: I0a9a4fe7a5395ca76135ceead05cbc3ee32b3d3c
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

cmd/containerboot/healthz.go

@@ -47,10 +47,10 @@ func (h *healthz) update(healthy bool) {
 	h.hasAddrs = healthy
 }
 
-// healthHandlers registers a simple health handler at /healthz.
+// registerHealthHandlers registers a simple health handler at /healthz.
 // A containerized tailscale instance is considered healthy if
 // it has at least one tailnet IP address.
-func healthHandlers(mux *http.ServeMux, podIPv4 string) *healthz {
+func registerHealthHandlers(mux *http.ServeMux, podIPv4 string) *healthz {
 	h := &healthz{podIPv4: podIPv4}
 	mux.Handle("GET /healthz", h)
 	return h