health: add Tracker type, in prep for removing global variables

This moves most of the health package global variables to a new
`health.Tracker` type.

But then rather than plumbing the Tracker in tsd.System everywhere,
this only goes halfway and makes one new global Tracker
(`health.Global`) that all the existing callers now use.

A future change will eliminate that global.

Updates #11874
Updates #4136

Change-Id: I6ee27e0b2e35f68cb38fecdb3b2dc4c3f2e09d68
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/localapi.go

@@ -358,7 +358,7 @@ func (h *Handler) serveBugReport(w http.ResponseWriter, r *http.Request) {
 	}
 	hi, _ := json.Marshal(hostinfo.New())
 	h.logf("user bugreport hostinfo: %s", hi)
-	if err := health.OverallError(); err != nil {
+	if err := health.Global.OverallError(); err != nil {
 		h.logf("user bugreport health: %s", err.Error())
 	} else {
 		h.logf("user bugreport health: ok")