Directories should be in package file list, not created by post-install hook

Directories like /etc/maxscale.cnf.d are not seen in RPM file lists, they are not created via the RPM package itself (via CMake / CPack INSTALL(DIRECTORY ...) but generated by explicit mkdir instructions in the post-install hook script.

That way they don't show up in rpm -ql list output, and it is not possible to find out what package they actually belong to via e.g. rpm -qf /etc/maxscale.cnf.d.

Also it's not possible to have rpm --setguids maxscale fix any permission issues on them, etc ...

Fix: use INSTALL(DIRECTORY ...) to create directories like the MariaDB-server RPM does.