wgengine, ipn: include number of active DERPs in status

Use this when making the ipn state transition from Starting to
Running. This way a network of quiet nodes with no active
handshaking will still transition to Active.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

ipn/backend.go

@@ -33,6 +33,7 @@ func (s State) String() string {
 type EngineStatus struct {
 	RBytes, WBytes wgengine.ByteCount
 	NumLive        int
+	LiveDERPs      int // number of active DERP connections
 	LivePeers      map[tailcfg.NodeKey]wgengine.PeerStatus
 }