Flush (remove) also hidden files

Due to an unpublished hack, I had a .htaccess file lying around in one of the imagecache preset directories. Using glob($path .'/*') didn't catch that one, because normally shells don't display hidden files with the default '*' pattern. That in turn causes imagecache to display a large white screen of PHP errors, which is baad.

Therefore, I did an alternative implementation using opendir/readdir/closedir instead of the glob function. Slightly larger but also works for people with hidden files. (I could also imagine someone uploading images with a leading dot... although... maybe not... well, it's your decision.)