ACLs on Linux: use acl_to_any_text for better speed

[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, using TEXT_NUMERIC_IDS
• 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, use a LRU caches of size 1000 to map uids to user names and to map gids to group names.

[ThomasWaldmann]

#9143 needs some review and testing.

If one has a network filesystem to back up and (network) uid->name lookups are slow, that should be much faster than before.