wgengine: simplify, change some signatures

* make RouterGen return an error, not take both tunname and tundev
* also remove RouteGen taking a wireguard/device.Device; currently unused
* remove derp parameter (it'll work differently)
* unexport NewUserspaceRouter in per-OS impls, add documented wrapper

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

wgengine/router_darwin.go

@@ -14,11 +14,12 @@ type darwinRouter struct {
 	tunname string
 }
 
-func NewUserspaceRouter(logf logger.Logf, tunname string, dev *device.Device, tuntap tun.Device, netChanged func()) Router {
-	r := darwinRouter{
-		tunname: tunname,
+func newUserspaceRouter(logf logger.Logf, _ *device.Device, tundev tun.Device, netChanged func()) (Router, error) {
+	tunname, err := tundev.Name()
+	if err != nil {
+		return nil, err
 	}
-	return &r
+	return &darwinRouter{tunname: tunname}, nil
 }
 
 func (r *darwinRouter) Up() error {