pakfire: Better errorhandling on downloads

pakfire: Better errorhandling on downloads

- Add true/false return codes to fetchfile, getmetafile and getmirrors
  indicating succes or failure.
- Check on those return codes and fail gracefully with clean
  error message(s) when downloads fail.
- Replace duplicate meta-file fetching code in dbgetlist with
  getmetafile function (fixing possibly missed cariage return
  conversion in meta-files).
- Remove pointless 5 retries to download server-list.db in
  selectmirror as fetchfile already retries 5 times.