net/tstun: return the real interface name at device creation.

This is usually the same as the requested interface, but on some
unixes can vary based on device number allocation, and on Windows
it's the GUID instead of the pretty name, since everything relating
to configuration wants the GUID.

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

net/tstun/tun_windows.go

@@ -8,6 +8,7 @@ import (
 	"github.com/tailscale/wireguard-go/tun"
 	"github.com/tailscale/wireguard-go/tun/wintun"
 	"golang.org/x/sys/windows"
+	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 )
 
 func init() {
@@ -22,3 +23,11 @@ func init() {
 	}
 	tun.WintunStaticRequestedGUID = &guid
 }
+
+func interfaceName(dev tun.Device) (string, error) {
+	guid, err := winipcfg.LUID(dev.(*tun.NativeTun).LUID()).GUID()
+	if err != nil {
+		return "", err
+	}
+	return guid.String(), nil
+}