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>

ipn/ipnlocal/local.go

@@ -2548,6 +2548,12 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	}
 	b.maybePauseControlClientLocked()
 
+	if nm != nil {
+		health.SetControlHealth(nm.ControlHealth)
+	} else {
+		health.SetControlHealth(nil)
+	}
+
 	// Determine if file sharing is enabled
 	fs := hasCapability(nm, tailcfg.CapabilityFileSharing)
 	if fs != b.capFileSharing {