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
My concern there was whether anyone currently uses the page size on Windows. Technically it would be a breaking change to start giving a different answer.