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
What is szfile? Looks like it used to be a pointer to the last byte, and now is a length?