du: Flags 'm', 'k', 'm' should be POSIX style overriden

[cerdelen]

The flags for Block sizes ('m', 'b', 'k') are being overridden in GNU.

Fix: #10661

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/shuf/shuf-reservoir (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/sort/sort-stale-thread-mem (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)

[cerdelen]

@spholz I wrote a function checking for the index of the flag 3 flags if present and applying the one with the highest index. The --apparent-size flag which is being set by -b is not affected by that as clap still parses it as present.

[spholz]

Thanks! du -s -b -m works as expected now.