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>

wgengine/wgengine.go

@@ -32,6 +32,7 @@ type PeerStatus struct {
 type Status struct {
 	Peers      []PeerStatus
 	LocalAddrs []string // TODO(crawshaw): []wgcfg.Endpoint?
+	DERPs      int      // number of active DERP connections
 }
 
 // StatusCallback is the type of status callbacks used by