Rewrote canon_usb_dialog() to be forgiving of replies that aren't
the length we expect. We now try to check the length reported
by the camera and use that when available.
Added some more code that some day will make the newer cameras
(e.g. EOS 20D) work, I hope.
git-svn-id: https://svn.code.sf.net/p/gphoto/code/trunk/libgphoto2@7615 67ed7778-7388-44ab-90cf-0a291f65f57c