bug#11557: cannot view nonempty *.x file using emacs (Image[imagemagick])

[Glenn Morris]

I've added an option imagemagick-type-enable with a conservative default.
I didn't put much thought into the default value.
I also wondered if for symmetry imagemagick-types-inhibit should maybe
be renamed to imagemagick-type-disable (or use -include and -exclude).

If anyone wants to change either of those it's fine by me.