Combine headerscheck and cpluspluscheck scripts

Combine headerscheck and cpluspluscheck scripts

They are mostly the same, and it is tedious to maintain two copies of
essentially the same exclude list.  headerscheck now has a new option
--cplusplus to select the cpluspluscheck functionality.  The top-level
make targets are still the same.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/4754a5b0-a32b-4036-a99a-6de14cf9fd72@eisentraut.org