Usage: images_equal [-all] fileA fileB

* Simple program to test if 2 2D images are identical.
* Exit status is 1 if they are equal, and 0 if they are not.
* If either image cannot be read, then exit status also 0.
* If the '-all' option is used, then all the images in the files
  are compared, and all must be equal for the exit status to be 1.
* If '-all' is NOT given, only the first image in each file is
* This program is meant for use in scripts that deal with DICOM
  servers that sometimes deal out multiple copies of the same
  image in different filenames :-(
* Also see program uniq_images, which works on multiple inputs.