[m-rev.] for review: start using the new code in file_names.m

[Zoltan Somogyi]

For review by Julien; the diff has an XXX marked for you.

I have bootchecked this in hlc.gc and csharp grades with
only the expected number of test case failures; I just started
a java bootcheck. However, this is the kind of change I would
like to have tested fairly thoroughly, which is why

- the diff sets things up that by default, the compiler checks
  the old and new filename construction algorithms against
  each other, and aborts if they differ, but

- the abort can be avoided by setting the environment variable
  named in the log message.

I intend to inspect the effect of this diff, once committed, on
the nightly tests for any failures caused by such aborts.

After a couple of weeks with no reported problems, I will delete the
old code that the new code is intended to replace.

Zoltan.
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