ipn/ipnserver, cmd/tailscaled: fix peerapi on Windows

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>
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