backend libpq void * argument for binary data

backend libpq void * argument for binary data

Change some backend libpq functions to take void * for binary data
instead of char *.  This removes the need for numerous casts.

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