net/netns: add windows support.

Also remove rebinding logic from the windows router. Magicsock will
instead rebind based on link change signals.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/interfaces/interfaces_windows.go

@@ -8,8 +8,10 @@ import (
 	"os/exec"
 	"syscall"
 
+	"github.com/tailscale/winipcfg-go"
 	"go4.org/mem"
 	"inet.af/netaddr"
+	"tailscale.com/tsconst"
 	"tailscale.com/util/lineread"
 )
 
@@ -71,3 +73,22 @@ func likelyHomeRouterIPWindows() (ret netaddr.IP, ok bool) {
 	})
 	return ret, !ret.IsZero()
 }
+
+// NonTailscaleMTUs returns a map of interface LUID to interface MTU,
+// for all interfaces except Tailscale tunnels.
+func NonTailscaleMTUs() (map[uint64]uint32, error) {
+	ifs, err := winipcfg.GetInterfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	ret := map[uint64]uint32{}
+	for _, iface := range ifs {
+		if iface.Description == tsconst.WintunInterfaceDesc {
+			continue
+		}
+		ret[iface.Luid] = iface.Mtu
+	}
+
+	return ret, nil
+}