ipn/{ipn,ipnlocal}: add per-user policy snapshots to IPN bus (#20135)

This adds the NotifyInitialPolicy watch option and the Policy field in
Notify so that clients can receive the effective policy snapshot via IPN
bus.

This extends policyclient.Client so ipnlocal can get and watch policy
snapshots, which is used by sysPolicyChanged to notify watchers.

User-scoped policy store registration, management, and cleanup will be
added in a follow-up

Updates tailscale/corp#42259

Signed-off-by: kari <kari@tailscale.com>

net/dns/manager_windows.go

@@ -78,7 +78,7 @@ func NewOSConfigurator(logf logger.Logf, health *health.Tracker, bus *eventbus.B
 	}
 
 	var err error
-	if ret.unregisterPolicyChangeCb, err = polc.RegisterChangeCallback(ret.sysPolicyChanged); err != nil {
+	if ret.unregisterPolicyChangeCb, err = polc.RegisterChangeCallback("", ret.sysPolicyChanged); err != nil {
 		logf("error registering policy change callback: %v", err) // non-fatal
 	}