in apache frontend, we have been using:

```
LogFormat "%h %l %{REMOTE_USER}i %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %D" combined
```

The %l is (from mod_log_config docs): Remote logname (from identd, if
supplied). This will return a dash unless mod_ident is present and
IdentityCheck is set On.

In the case of apache frontend, it was always a - . This is missing in
caddy frontend and our existing log processing tools (apachedex) cannot
be used on frontend logs since we switched to Caddy.

/reviewed-on nexedi/slapos!530