Otherwise it would log warnings about an empty file.
```
stores.go:138: store.NewFileStore("/tmp/3777352782"): file empty; treating it like a missing file [warning]
```
Signed-off-by: Maisem Ali <maisem@tailscale.com>
Also move KubeStore and MemStore into their own package.
RELNOTE: tsnet now supports providing a custom ipn.StateStore.
Signed-off-by: Maisem Ali <maisem@tailscale.com>
Brad Fitzpatrick
Maisem Ali