[m-rev.] for review: unify getopt and getopt_io

[Zoltan Somogyi]

For review by anyone, though I would like both Peter and Julien
to have a look at the concept, and at the additional help for
diagnosing breaking changes.

I intend to post a diff for NEWS tomorrow, covering both this change,
and other recent changes to getopt*.m.

Zoltan.
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