icasefs: rewrite comment to explain situtation precisely

icasefs: follow standard cache look up pattern

icasefs: disuse length check against un-normcase()-ed filenames this patch disuses length check against un-normcase()-ed filenames gotten by "os.listdir()", because there is no assurance that filesystem stores filenames normalized except in letter case, even though some case insensitive filesystems (in some environment, for some language setting) store them in such manner.