[Rd] Use of restricted c++ keywords as variable names in headers

[Travers Ching]

I was trying to use one of the headers in R_ext/, but had trouble.  I
determined that it was due to using restricted keywords as variable names.
So to load in the header, I needed to do this:

#define class klass
#define private krivate
#include <R_ext/Connections.h>
#undef class
#undef private

I know that the altrep.h header previously had the same issue, but was
fixed.  Could this be changed as well?

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