perform copy_file_range until EOF is reached instead of basing things on file size

This solves several problems

- race conditions where a file is truncated while copying from it. if we blindly trusted
  the file size this would lead to an infinite loop
- proc files appearing empty to copy_file_range but not to read/write
  https://github.com/coreutils/coreutils/commit/4b04a0c
- copy_file_range returning 0 for some filesystems (overlay? bind mounts?)
  inside docker, again leading to an infinite loop

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