Revert "Do not automatically use MAP_POPULATE for opening mmap files."

This reverts commit cba75db4e36b3af0d39b5717141a971e6d558554. The prior
commit yields a compiler error 'error: lvalue required as left operand
of assignment; MAP_POPULATE = ((int) 0);'. So for now this branch
*always* adds MAP_POPULATE when using the open and mapMem procs.