Don't set `auto-image-file-mode'. Do not create the thumbnails directory on

Don't set `auto-image-file-mode'.  Do not create the thumbnails directory on
loading.
(thumbs-conversion-program): Use `eq' to check the system type, not `equal'.
(thumbs-temp-dir): Set to `temporary-file-directory', not "/tmp".  Fix
docstring.
(thumbs-thumbsdir): New function to return the thumbnails directory, creating it
if needed.
(thumbs-cleanup-thumbsdir, thumbs-thumbname): Use it.
(thumbs-temp-file): Delete variable and make it into a function.
(thumbs-resize-image, thumbs-modify-image): Use it.
(thumbs-kill-buffer): Simplify.
(thumbs-gensym): Defalias or duplicate CL `gensym'.
(thumbs-resize-image, thumbs-resize-interactive): Fix typos in docstrings.