ipn: don't Logout when Windows GUI disconnects

Logout used to be a no-op, so the ipnserver previously synthensized a Logout
on disconnect. Now that Logout actually invalidates the node key that was
forcing all GUI closes to log people out.

Instead, add a method to LocalBackend to specifically mean "the
Windows GUI closed, please forget all the state".

Fixes tailscale/corp#1591 (ignoring the notification issues, tracked elsewhere)

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