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".
Fixestailscale/corp#1591 (ignoring the notification issues, tracked elsewhere)
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>