More Windows installer improvements (#607)

- `Uninstall-SecurityDaemon` now defaults to leaving config.yaml and
  Moby data root, unless new flags `-DeleteConfig` and `-DeleteMobyDataRoot`
  are specified.

- `Uninstall-SecurityDaemon` now deletes the edge containers instead of just
  stopping them, to ensure that they're deleted even if the Moby data root
  isn't.

- `Install-SecurityDaemon` now has a new flag `-ExistingConfig` that can be
  set to use an existing config.yaml, instead of generating a new one via
  `-Manual` or `-Dps`

- Fixed a bug in `Uninstall-SecurityDaemon` where it incorrectly parsed the
  result of `docker inspect`.

- Add comment-based help, and fix two more cases of Moby Runtime -> Moby Engine.