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_linux.go

@@ -122,17 +122,18 @@ func likelyHomeRouterIPAndroid() (ret netaddr.IP, ok bool) {
 	return ret, !ret.IsZero()
 }
 
-// DefaultRouteInterface returns the name of the network interface that owns
-// the default route, not including any tailscale interfaces.
-func DefaultRouteInterface() (string, error) {
+func defaultRoute() (d DefaultRouteDetails, err error) {
 	v, err := defaultRouteInterfaceProcNet()
 	if err == nil {
-		return v, nil
+		d.InterfaceName = v
+		return d, nil
 	}
 	if runtime.GOOS == "android" {
-		return defaultRouteInterfaceAndroidIPRoute()
+		v, err = defaultRouteInterfaceAndroidIPRoute()
+		d.InterfaceName = v
+		return d, err
 	}
-	return v, err
+	return d, err
 }
 
 var zeroRouteBytes = []byte("00000000")