wgengine/router: remove wireguard-go config from settings.

Instead, pass in only exactly the relevant configuration pieces
that the OS network stack cares about.

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

wgengine/router/router_fake.go

@@ -25,8 +25,8 @@ func (r fakeRouter) Up() error {
 	return nil
 }
 
-func (r fakeRouter) SetRoutes(rs RouteSettings) error {
-	r.logf("Warning: fakeRouter.SetRoutes: not implemented.")
+func (r fakeRouter) Set(rs Settings) error {
+	r.logf("Warning: fakeRouter.Set: not implemented.")
 	return nil
 }