deb_download_unpack.sh: close #17; script is working

- Added `--dry-run` and `--download-only options`
- Added the guts of the `--dry-run` functionality
- Renamed `--source` to `--host-arch`, and `--target` to `--target-arch`
- Set `--host-arch` via a query to `dpkg` if it's not used on the command line
- Add `eval` to the calls to `wget` to get the params set up correctly