mm-util.el: Just return the image directories, not all directories in the path in...

mm-util.el: Just return the image directories, not all directories in the path in addition to the image directories; Maintain a cache of the image directories.  This means that the `g' command in Gnus doesn't have to stat dozens of directories each time; nnmh.el: Only recurse down into subdirectories if the link count is more than 2.  This results in a 100x speed up on my nnmh spool, and that's from an SSD disk, and not over nfs.