MemFiles on win32 and win64

[jlp765]

For memfiles, I believe there are issues on windows platform with trying to 
open a file again after a close.

(just scoping the size of the problem before raising a PR)

**Can someone please test this on win32?** (I know it fails on win64)

Also test **with/without** the compiler setting `-d:release`
    
    
    import os, memfiles, times
    
    proc runTst1(fname: string) =
      var
        file = memfiles.open(fname, fmRead)
        i = 0
        t0 = epochTime()
      for field in lines(file):
        inc(i)
      memfiles.close(file)
      echo "Read lines ", i, " lines! ", epochTime() - t0
    
    proc main() =
      if paramCount() < 1:
        quit("ERROR: Please provide a file name for read testing")
      runTst1(paramStr(1))
      runTst1(paramStr(1))
    main()