In opr_sanity regression test, check for unexpected uses of cstring.

In light of commit ea0d494dae0d3d6f, it seems like a good idea to add
a regression test that will complain about random functions taking or
returning cstring.  Only I/O support functions and encoding conversion
functions should be declared that way.

While at it, add some checks that encoding conversion functions are
declared properly.  Since pg_conversion isn't populated manually,
it's not quite as necessary to check its contents as it is for catalogs
like pg_proc; but one thing we definitely have not tested in the past
is whether the identified conproc for a conversion actually does that
conversion vs. some other one.

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