- implement use of different intervals as source for cloning
   * use 'woechentlich' if available and no backup exists for 'daily'
   * add 'prefer_latest' to prefer different interval, that's newer than
     ours
   * or add 'prefer_same_interval' instead?
- implement detection of partial backups
- do not check by name, but by time
   * is ls -t posix?
   * also check: -p -1