Combine headerscheck and cpluspluscheck scripts

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.