[m-rev.] for review: make warning/info messages conditional on an option

[Zoltan Somogyi]

This implements most of the proposal from a day or two ago.
For review by anyone, but I would like Peter to write the help text
for the new option warn_opt_desc_spec, since it applies to diagnostics
in code that he touched last. The option may need renaming as well.

I would like feedback on the names of the new options.
I would also like feedback on the two ZZZs in options.m,
which I intend to act on after this diff is committed.
A third ZZZ is a reminder to me to move a predicate
whose movement before commit would make the diff
harder to review.

This diff does not define subtypes of "option" to be used
as the argument types of severity_{warning,informational}.
The right place to define those subtypes would be in error_spec.m.
In the absence of those subtypes, several modules have to
import options.m, which they did not import before.
If we added those subtypes, these new imports would
not be needed, but every module that imports the error_spec
module would have a significantly bigger parse_tree.error_spec.int
to read in. I think these two performance effects probably
offset each other. I chose not to add the subtypes because
they would represent a maintenance burden, and I don't think
they would bring enough advantage to compensate for that.
However, I am willing to entertain other opinions.

Zoltan.
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