Redesign interaction between line-end translation and changes/undo

Redesign interaction between line-end translation and changes/undo
recording.  New model suppresses changes and undo only for output
translations, since these translations will be undone before the
redisplay occurs.  When inserting a file into the buffer, it does not
affect the change recording at all, since the translation won't exceed
the boundaries of the inserted text, and it's desirable to notice the
change of inserting the file in the usual way.