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>

net/tlsdial/tlsdial_test.go

@@ -15,6 +15,8 @@ import (
 	"runtime"
 	"sync/atomic"
 	"testing"
+
+	"tailscale.com/health"
 )
 
 func resetOnce() {
@@ -105,7 +107,8 @@ func TestFallbackRootWorks(t *testing.T) {
 		},
 		DisableKeepAlives: true, // for test cleanup ease
 	}
-	tr.TLSClientConfig = Config("tlsdial.test", tr.TLSClientConfig)
+	ht := new(health.Tracker)
+	tr.TLSClientConfig = Config("tlsdial.test", ht, tr.TLSClientConfig)
 	c := &http.Client{Transport: tr}
 
 	ctr0 := atomic.LoadInt32(&counterFallbackOK)