Clearing caches fail if cache folder is symlinked

if any of the cache folders under ezp_root/var/ is symlinked to a dir outside the ezp_root, no cache will ever be cleaned. the ezDir::recursiveDelete() checks that the absolute path is outside the root and fails.

test case:

• mv ezpublish_legacy/var/cache /some/path/elsewhere
• ln -s /some/path/elsewhere ezpublish_legacy/var/cache
• clear caches from admin or command line
• inspect the symlinked folder, all files all still there

there are no errors in log, but hacking the recursiveDelete() to write a message with the result shows clearly that the return value is False.