fai-savelog: upload logs in chroot jail

Hi there,

Only recently I managed to switch to Debian Bookworm and FAI 6.0.3.

So far, on the FAI server side, I used a shell script to validate and 
restrict the log upload of fai-savelog.

In particular, the user `fai` for login on the server has a custom 
script (fai-shell) as its login shell. My `authorized_keys` has entries 
like these:

command="egon" ssh-ed25519 AAAAC3Nza…

Now, ssh runs fai-shell with "egon" as command and stores the 
client-supplient command in $SSH_ORIGINAL_COMMAND. In that way, 
fai-shell can validate that a client using this particular SSH key may 
only write into log/egon/softupdate-….

However, recent ssh versions switched to sftp internally, so now 
$SSH_ORIGINAL_COMMAND only contains "internal-sftp". Is there a clever 
way to enforce a client separation like I have with older SSH versions? 
Ideally, a client may only write into his client-specific directory but 
neither read anything nor write to another directory.

The current implementation in fai-savelog unfortunately complicates a 
solution with a chroot jail like in [1] as it first runs `mkdir` and 
`ln`s. But even using sftp commands only would require a decent amount 
of prepartion on the server side (effectively one chroot per client or a 
virtual sftp implementation).

My current workaround is to manually upload the logs with scp's -O flag 
(i.e., to use the legacy scp protocol) in a hook.

Best,
Maximilian

[1]: https://linuxize.com/post/how-to-set-up-sftp-chroot-jail/