functions with ntdll equivalent
- replaced status setter/getter for wine async structures with direct
  access to a (now included) IO_STATUS_BLOCK structure
- since we now have a IO_STATUS_BLOCK in async_private, we no longer
  need in most of the user (derivated) structures a field for
  LPOVERLAPPED (it's stored as the IO_STATUS_BLOCK)
- rewrote the async.h users accordingly
- implemented ntdll.Nt{Read|Write}File and let
  kernel32.{Read|Write}File(Ex)? use those new ntdll functions
- rewrote smb read/write interfaces to be more ntdll stylish (no
  overlapped yet)