make local style.ini ready for farms

This PR is not complete yet. I opened it anyway so that it can be discussed and/or picked up on by someone else.

This fixes the issue that local style.ini's cannot be set per conf folder, the usual place for a child wiki in a wiki farm. But it only fixes it for `[replacements]` and not yet for `[stylesheets]`.

With this patch a style.local.ini file can be stored in the animal's conf folder, e.g. `conf/tpl/dokuwiki/style.local.ini`.