Fall back to ~/.config/ if that isn't set.

- Store parameters in ~/.config/Mageia/dorsync.conf. When dorsync is
  executed for the first time, it creates this config file if not found.
- If dorsync.skip is missing, dorsync asks the user if he really wants to
  download all ISOs or if he wants to select which ones to download, and
  stores the user choice in dorsync.skip for future reuse.
- Update README.md

Replace sha1sum by sha3-512sum and fix arguments passed to rsync.

This simplifies the script slightly and makes it more resilient to
unexpected data.

The output of udisks and udisksctl are different, and they do not
display the same fields. Now use udevadm to get the missing data.