headerscheck started in 55ea1091884 (2019) essentially as an adjusted
copy of cpluspluscheck. Since then two scripts have not drifted far
apart. But there are occasionally mistakes keeping the two exclude
lists updated together. I figure we can just combine the two scripts
into one, so it's easier to keep updated.
The attached patch adds an option --cplusplus to headerscheck, with
which it does the same thing as cpluspluscheck, and cpluspluscheck is
removed. The top-level make targets stay the same.
