(image-file-name-extensions): New variable.

(image-file-name-extensions): New variable.
(image-file-name-regexps): Renamed from `image-file-regexps'.
  New default value is nil.  Call `auto-image-file-mode'.
(image-file-name-regexp): New function.
(auto-image-file-mode): New minor mode.
(insert-image-file): Don't make conditional on the image-file
  handler being enabled.
(image-file-handler): Make the call here conditional instead.
(set-image-file-handler-enabled, enable-image-file-handler)
(disable-image-file-handler): Functions removed.