net/interfaces: define DefaultRouteInterface and State.DefaultRouteInterface

It was pretty ill-defined before and mostly for logging. But I wanted
to start depending on it, so define what it is and make Windows match
the other operating systems, without losing the log output we had
before. (and add tests for that)

Change-Id: I0fbbba1cfc67a265d09dd6cb738b73f0f6005247
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/interfaces/interfaces_windows.go

@@ -5,7 +5,6 @@
 package interfaces
 
 import (
-	"fmt"
 	"log"
 	"net"
 	"net/url"
@@ -217,18 +216,20 @@ func GetWindowsDefault(family winipcfg.AddressFamily) (*winipcfg.IPAdapterAddres
 	return bestIface, nil
 }
 
-func DefaultRouteInterface() (string, error) {
+func defaultRoute() (d DefaultRouteDetails, err error) {
 	// We always return the IPv4 default route.
 	// TODO(bradfitz): adjust API if/when anything cares. They could in theory differ, though,
 	// in which case we might send traffic to the wrong interface.
 	iface, err := GetWindowsDefault(windows.AF_INET)
 	if err != nil {
-		return "", err
+		return d, err
 	}
-	if iface == nil {
-		return "(none)", nil
+	if iface != nil {
+		d.InterfaceName = iface.FriendlyName()
+		d.InterfaceDesc = iface.Description()
+		d.InterfaceIndex = int(iface.IfIndex)
 	}
-	return fmt.Sprintf("%s (%s)", iface.FriendlyName(), iface.Description()), nil
+	return d, nil
 }
 
 var (