Atomize output files

Atomize output files

I feel like I need to relearn signals every time I have to interact with
them. Best get this done while the iron's hot.

1. The ROA file is first written as `<cache>/.roa`.
   The RK file is first written as `<cache>/.rk`.
2. When the validation run is done, `.roa` is renamed to `--output.roa`,
   and `.rk` becomes `--output.bgpsec`.
3. Most terminating signals unlink `.roa` and `.rk`.

Fixes #124.