nixos: refactor tarsnap backup service module

Major changes
- Port to systemd timers: for each archive configuration is created a
  tarsnap@archive-name.timer which triggers the instanced service unit
- Rename the `config` option to `archives`

Minor/superficial improvements
- Restrict tarsnap service capabilities
- Use dirOf builtin
- Set executable bit for owner of tarsnap cache directory
- Set IOSchedulingClass to idle
- Humanize numbers when printing stats
- Rewrite most option descriptions
- Simplify assertion