don't use MAKE_PTR/OFFSET for shmem pointers

Since we require every process to map the shared memory region to the same 
address, we don't need the MAKE_PTR/OFFSET code that was needed when that
was not the case.  This patch makes shared memory pointers just like 
regular pointers.

http://archives.postgresql.org/pgsql-general/2007-08/msg01510.php

Kris Jurka