Fix mmap_array with offset on Windows

query allocation granularity (64k on Windows, larger than page size which is 4k),
ensure offset to MapViewOfFile is a multiple of the granularity

add a test for mmap_array with offset

fixes #7080 and #6203

some mmap issues remain with HDF5.jl, but there may be something else going on there
dwMaximumSize - quoting [MSDN](http://msdn.microsoft.com/en-us/library/windows/desktop/aa366537(v=vs.85).aspx), "maximum size of the file mapping object"

You probably know more about this stuff than I do, I'm not 100% sure I did this right. Still some trouble with HDF5, and when I tried a large offset, but at least this was able to fix the simple case that's currently broken.