Allow binary I/O of type "void".

Allow binary I/O of type "void".

void_send is useful for the same reason that void_out doesn't throw error,
namely that someone might do "select void_returning_func(...)"  from a
client that prefers to operate in binary mode.  The void_recv function may
or may not have any practical use, but we provide it for symmetry.

Radosław Smogura