tsd, ipnlocal, etc: add tsd.System.HealthTracker, start some plumbing

This adds a health.Tracker to tsd.System, accessible via
a new tsd.System.HealthTracker method.

In the future, that new method will return a tsd.System-specific
HealthTracker, so multiple tsnet.Servers in the same process are
isolated. For now, though, it just always returns the temporary
health.Global value. That permits incremental plumbing over a number
of changes. When the second to last health.Global reference is gone,
then the tsd.System.HealthTracker implementation can return a private
Tracker.

The primary plumbing this does is adding it to LocalBackend and its
dozen and change health calls. A few misc other callers are also
plumbed. Subsequent changes will flesh out other parts of the tree
(magicsock, controlclient, etc).

Updates #11874
Updates #4136

Change-Id: Id51e73cfc8a39110425b6dc19d18b3975eac75ce
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsd/tsd.go

@@ -23,6 +23,7 @@ import (
 
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/drive"
+	"tailscale.com/health"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/conffile"
 	"tailscale.com/net/dns"
@@ -63,6 +64,8 @@ type System struct {
 
 	controlKnobs controlknobs.Knobs
 	proxyMap     proxymap.Mapper
+
+	healthTracker health.Tracker
 }
 
 // NetstackImpl is the interface that *netstack.Impl implements.
@@ -134,6 +137,19 @@ func (s *System) ProxyMapper() *proxymap.Mapper {
 	return &s.proxyMap
 }
 
+// HealthTracker returns the system health tracker.
+func (s *System) HealthTracker() *health.Tracker {
+	// TODO(bradfitz): plumb the tsd.System.HealthTracker() value
+	// everywhere and then then remove this use of the global
+	// and remove health.Global entirely. But for now we keep
+	// the two in sync during plumbing.
+	const stillPlumbing = true
+	if stillPlumbing {
+		return health.Global
+	}
+	return &s.healthTracker
+}
+
 // SubSystem represents some subsystem of the Tailscale node daemon.
 //
 // A subsystem can be set to a value, and then later retrieved. A subsystem