[m-rev.] for review: pragma format_call

[Zoltan Somogyi]

For review by anyone.

I have not made any changes to the reference manual since addressing
the review of its diff.

This diff has a ZZZ in add_pragma.m, on code that checks the status
of the new pragma the same way that we check the status of other
declarative pragmas: we generate an error if the predicate (or function)
to which the pragma applies is exported, but the pragma itself is not exported.

I think this test is wrong. We should generate an error in the *opposite*
case: the pragma is exported, but the pred/func it applies to is not exported.
The case when the pred/func is exported but the declarative pragma is not
exported should merit only a warning, because the other modules that
import that pred/func can survive without knowing that e.g. that pred/func
is obsolete, or that calls to it can be checked the same way as calls to e.g.
string.format.

Opinions?

Zoltan.
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