`define-minor-mode' actually calls the mode-function if the associated
variable is non-nil, which requires that all needed functions be already
defined.
(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.
