Squashed commit of the following 6779-install-log-dir branch:

commit b31ac616
Author: Herman van Rink <rink@initfour.nl>
Date:   Fri Oct 25 17:08:45 2024 +0200

    restore (seemingly redundant) mkdir

commit 9a986872
Author: Herman van Rink <rink@initfour.nl>
Date:   Fri Oct 25 16:37:58 2024 +0200

    Move define and mkdir earlier in the script

commit 6eef0af5
Author: Herman van Rink <rink@initfour.nl>
Date:   Fri Jan 13 22:54:17 2023 +0100

    Move install.log into the ispconfig_log_dir