(enum tiff_keyword_index, tiff_format): Add :index keyword.

(enum tiff_keyword_index, tiff_format): Add :index keyword.
(fn_TIFFSetDirectory): New library function used.
(init_tiff_functions) [HAVE_NTGUI]: Initialize it.
(tiff_load): Use :index to select among multiple images.  Set count
property when multiple images exist.
(gif_format): Use :index, not :image.