feature: du shared extents reflink

[cre4ture]

Addresses #3906:

On copy-on-write filesystems it is possible to copy files without really copying the contents. The contents are copied when modified.

This pullrequests introduces a new feature for "du --shared-extents" that consideres the shared extents of the files, not counting it double.

[cre4ture]

As said in the ticket, i don't want to add new features that GNU doesn't have until we are at parity. Sorry.

I see. So what do we want to do with this pull request? I've seen you started commenting already. Thanks for this :-) . But if its not gona be merged, we should stop working on it. Whats your plan?

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)