tailcfg, health: add way for control plane to add problems to health check

So if the control plane knows that something's broken about the node, it can
include problem(s) in MapResponse and "tailscale status" will show it.
(and GUIs in the future, as it's in ipnstate.Status/JSON)

This also bumps the MapRequest.Version, though it's not strictly
required. Doesn't hurt.

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

health/health.go

@@ -40,6 +40,7 @@ var (
 	ipnWantRunning          bool
 	anyInterfaceUp          = true // until told otherwise
 	udp4Unbound             bool
+	controlHealth           []string
 )
 
 // Subsystem is the name of a subsystem whose health can be monitored.
@@ -141,6 +142,13 @@ func setLocked(key Subsystem, err error) {
 	}
 }
 
+func SetControlHealth(problems []string) {
+	mu.Lock()
+	defer mu.Unlock()
+	controlHealth = problems
+	selfCheckLocked()
+}
+
 // GotStreamedMapResponse notes that we got a tailcfg.MapResponse
 // message in streaming mode, even if it's just a keep-alive message.
 func GotStreamedMapResponse() {
@@ -318,6 +326,9 @@ func overallErrorLocked() error {
 	for regionID, problem := range derpRegionHealthProblem {
 		errs = append(errs, fmt.Errorf("derp%d: %v", regionID, problem))
 	}
+	for _, s := range controlHealth {
+		errs = append(errs, errors.New(s))
+	}
 	if e := fakeErrForTesting; len(errs) == 0 && e != "" {
 		return errors.New(e)
 	}