cmd/tailscaled: add a special command to tailscaled's Windows service for removing WinTun

WinTun is installed lazily by tailscaled while it is running as LocalSystem.
Based upon what we're seeing in bug reports and support requests, removing
WinTun as a lesser user may fail under certain Windows versions, even when that
user is an Administrator.

By adding a user-defined command code to tailscaled, we can ask the service to
do the removal on our behalf while it is still running as LocalSystem.

* The uninstall code is basically the same as it is in corp;
* The command code will be sent as a service control request and is protected by
  the SERVICE_USER_DEFINED_CONTROL access right, which requires Administrator.

I'll be adding follow-up patches in corp to engage this functionality.

Updates https://github.com/tailscale/tailscale/issues/6433

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

tstest/integration/tailscaled_deps_test_windows.go

@@ -16,6 +16,7 @@ import (
 	_ "golang.org/x/sys/windows/svc"
 	_ "golang.org/x/sys/windows/svc/eventlog"
 	_ "golang.org/x/sys/windows/svc/mgr"
+	_ "golang.zx2c4.com/wintun"
 	_ "golang.zx2c4.com/wireguard/tun"
 	_ "golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	_ "tailscale.com/cmd/tailscaled/childproc"