[m-rev.] for review: mode-specific unused arg warnings

[Zoltan Somogyi]

For review by Julien, since part of this diff is his idea.

By the way, do any of you guys ever run the test cases using mmc --make
*in C grades*? The reason I ask is that the warnings/unused_args_analysis
test case is enabled ONLY with mmc --make, but it is also DISABLED in
java and C# grades. As far as I can tell, it has not been executed in a long time,
because its non-colorized expected output files would cause any execution
to fail.

Zoltan.
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