posixfs: use fadvise "dontneed"?

If a store user reads or writes a lot of data, caching might not be beneficial.

E.g. if a backup tool like borgbackup does a backup and writes huge amounts `packs/`, it is rather unlikely that it will read the packs again soon afterwards.

Similar for a restore: if it reads packs, it will often not need to read the same data from the packs again (some exceptions like the all-zero chunk will likely be cached in the backup program).