Add automatic MAP_POPULATE flag for opening read_only (MAP_PRIVATE) and shared (MAP_SHARED) mmap files.
Do not automatically use MAP_POPULATE for opening mmap files.

Adds use_map_populate keyword to memfiles.open and memfiles.mapMem
to govern MAP_POPULATE use. This is set to false by default.
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.
Only try to import MAP_POPULATE on Linux, define flag as 0 otherwise.
Merge pull request #1405 from boydgreenfield/add_mmap_map_populate

Add mmap map populate