This iteration improves error handling for copy_file_range,
particularly around undocumented error codes in earlier kernel
versions.
Additionally this fixes the userspace copy fallback to handle
zero-length files such as in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108178.

Lastly, the case "src gets resized during the copy loop" is now
considered and will return true once the loop hits EOF (this is the
only situation, aside from a zero-length src, where sendfile and
copy_file_range return 0).

Best
Jannik