ACLs on Linux: use acl_to_any_text

[ThomasWaldmann]

https://man7.org/linux/man-pages/man3/acl_to_any_text.3.html

Compared to acl_to_text, we can:

• get the uid / gid instead of the names
• avoid a gazillion more or less expensive, uncached lookups inside libacl for the user/group names (especially for samba / winbind this can be slow)
• replace these by cached lookups inside borg