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
from reading msdn, it appears that szfile here should = offset + szarray

or it can equal 0, without any actual cost (http://msdn.microsoft.com/en-us/library/windows/desktop/aa366542(v=vs.85).aspx)