For now, let us refrain from putting the sync mode into the Io.File
struct, and document that to do concurrent batch operations, any Windows
file handles must be in asynchronous mode. The consequences for
violating this requirement is neither illegal behavior, nor an error,
but that concurrency is lost. In other words, deadlock might occur. This
prevents the addition of flags field.

partial revert of 2faf14200f58ee72ec3a13e894d765f59e6483a9