- batchAwaitAsync does blocking reads with NtReadFile (no APC, no event)
  when the nonblocking flag is unset, but still takes advantage of
  APCs when nonblocking flag is set.
- batchAwaitConcurrent returns error.ConcurrencyUnavailable when it
  encounters a file_read_streaming operation on a file in blocking mode.
- fileReadStreaming avoids pointlessly checking sync cancelation status
  when nonblocking flag is set, uses an APC with a done flag, and waits
  on that value to change in NtDelayExecution before returning.
- fix incorrect use of NtCancelIoFile (ntdll function prototype was
  wrong, leading to misuse)