Rework configuration loading to use a predefined file list or cascade

This change add the global $config_cascade which holds the list of files to be
read for each configuration setting group.  Dokuwiki adds in its configuration
file values after preload.php, giving preload.php to set its own configuration
cascade.

One side effect of the change is "local.protected.php" is part of the default
cascade, removing the need for it to be included at the bottom of local.php.

darcs-hash:20090118181204-f07c6-fea1c406da1bbdb0a52ab40914f11b835e797728.gz