We weren't wiring up netstack.Impl to the LocalBackend in some cases
on Windows. This fixes Windows 7 when run as a service.
Updates #4750 (fixes after pull in to corp repo)
Change-Id: I9ce51b797710f2bedfa90545776b7628c7528e99
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>