[m-rev.] diff: cutting over to the new code in file_names.m

[Zoltan Somogyi]

The concept has been approved and the diff is relatively
simple, so I don't think a review is needed, but I wouldn't
object to one. I deliberately did not include two things
in this diff to make review easier.

- Renaming the newext type to just ext, doing the same
  edit on its function symbols.

- Restructuring file_names.m, moving the comments
  recording lessons learned to the places in the new code
  where they are relevant.

These will come in the next few ways.

Zoltan.
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