Prevent creating thumbnails of all gif frames

Prevent creating thumbnails of all gif frames

With the previous defaults, doing image-dired on a directory with an
animated foo.gif would cause creation of foo.thumb-N.gif for each of
N frames in foo.gif.  By default image-dired looks for foo.thumb.gif, so
there additionally is no usable thumbnail after all the needless effort.
image-dired never handled animation, regardless.
* lisp/image-dired.el: Mention limitation.
(image-dired-cmd-create-thumbnail-options):
(image-dired-cmd-create-temp-image-options):
(image-dired-cmd-create-standard-thumbnail-options): Append [0] to
filename to indicate only converting the 0th frame.
(image-dired-display-image-mode): Don't show a cursor.