Call imagemagick-register-types automatically.

Call imagemagick-register-types automatically.

* lisp/image.el (imagemagick--extension-regexp): New variable.
(imagemagick-register-types): Use it.
(imagemagick-types-inhibit): Add :set function.  Allow new value
of t to inhibit all types.

* lisp/loadup.el (fboundp): Preload regexp-opt, needed by
imagemagick-register-types.

* lisp/emacs-lisp/regexp-opt.el (regexp-opt-charset): Avoid cl macros,
so we can preload it.