wgengine/router{,/osrouter}: split OS router implementations into subpackage

So wgengine/router is just the docs + entrypoint + types, and then
underscore importing wgengine/router/osrouter registers the constructors
with the wgengine/router package.

Then tsnet can not pull those in.

Updates #17313

Change-Id: If313226f6987d709ea9193c8f16a909326ceefe7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/router/osrouter/router_freebsd.go

@@ -0,0 +1,27 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package osrouter
+
+import (
+	"tailscale.com/net/netmon"
+	"tailscale.com/types/logger"
+	"tailscale.com/wgengine/router"
+)
+
+func init() {
+	router.HookCleanUp.Set(func(logf logger.Logf, netMon *netmon.Monitor, ifName string) {
+		cleanUp(logf, ifName)
+	})
+}
+
+func cleanUp(logf logger.Logf, interfaceName string) {
+	// If the interface was left behind, ifconfig down will not remove it.
+	// In fact, this will leave a system in a tainted state where starting tailscaled
+	// will result in "interface tailscale0 already exists"
+	// until the defunct interface is ifconfig-destroyed.
+	ifup := []string{"ifconfig", interfaceName, "destroy"}
+	if out, err := cmd(ifup...).CombinedOutput(); err != nil {
+		logf("ifconfig destroy: %v\n%s", err, out)
+	}
+}