XLogRegisterData, XLogRegisterBufData void * argument for binary data

XLogRegisterData, XLogRegisterBufData void * argument for binary data

Change XLogRegisterData() and XLogRegisterBufData() functions to take
void * for binary data instead of char *.  This will remove the need
for numerous casts (done in a separate commit for clarity).

Reviewed-by: Dagfinn Ilmari Mannsåker 
Discussion: https://www.postgresql.org/message-id/flat/fd1fcedb-3492-4fc8-9e3e-74b97f2db6c7%40eisentraut.org