Image autodetection

[Chong Yidong]

I have tried to digest the long and confusing thread on this subject.
I propose the following:

1. Make a new variable, auto-detect-image-type-p.  If non-nil, the
   function image-type-auto-detected-p (called from magic-mode-alist)
   performs content detection.  The default is nil.

2. Do not display images as images automatically in image-mode.
   Instead, the user sees "Type C-c C-c to view the image as an
   image".  This additional step should not be a major inconvenience.

The other changes proposed in that thread, such as using
image-minor-mode by default, are probably unnecessary.