Fix EZP-22915: Clearing caches fail if cache folder is symlinked

https://jira.ez.no/browse/EZP-22915

In `eZDir::recursiveDelete()`, root check was added in c6c384545548a90c85f25d16352b06f2925f9a40 in order to fix [EZP-15823](https://jira.ez.no/browse/EZP-15823).
To be less restrictive, I added a new setting in `site.ini` allowing to specify which directory *outside* eZ Publish root dir can be considered as a valid *path prefix* for recursive deletion.
This line introduce an "egg or chicken" dilemma : when you use ezcache.php to clear the global ini cache, this line will read the old settings, and may disallow you to clear the cache.