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.

lib/pure/memfiles.nim

@@ -30,14 +30,17 @@ type
 
     when defined(windows):
       fHandle: int
-      mapHandle: int 
+      mapHandle: int
     else:
       handle: cint
 
 
 proc mapMem*(m: var TMemFile, mode: TFileMode = fmRead,
-             mappedSize = -1, offset = 0): pointer =
+             mappedSize = -1, offset = 0,
+             use_map_populate = false): pointer =
   var readonly = mode == fmRead
+  if not use_map_populate:
+    MAP_POPULATE = 0
   when defined(windows):
     result = mapViewOfFileEx(
       m.mapHandle,
@@ -72,7 +75,8 @@ proc unmapMem*(f: var TMemFile, p: pointer, size: int) =
 
 
 proc open*(filename: string, mode: TFileMode = fmRead,
-           mappedSize = -1, offset = 0, newFileSize = -1): TMemFile =
+           mappedSize = -1, offset = 0, newFileSize = -1,
+           use_map_populate = false): TMemFile =
   ## opens a memory mapped file. If this fails, ``EOS`` is raised.
   ## `newFileSize` can only be set if the file does not exist and is opened
   ## with write access (e.g., with fmReadWrite). `mappedSize` and `offset`
@@ -94,6 +98,8 @@ proc open*(filename: string, mode: TFileMode = fmRead,
   # The file can be resized only when write mode is used:
   assert newFileSize == -1 or mode != fmRead
   var readonly = mode == fmRead
+  if not use_map_populate:
+    MAP_POPULATE = 0
 
   template rollback =
     result.mem = nil