net/interfaces, net/netns: move default route interface code to interfaces

To populate interfaces.State.DefaultRouteInterface.

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

net/interfaces/interfaces_defaultrouteif_todo.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux
+// +build !linux,!windows
 
 package interfaces