Hi,

This PR fixes several issues:

1. The permissions for the pipes created through createPipe were accidentally swapped. The read end got write permissions while the write part got read permissions in Haskell. The permissions were correct in the underlying OS object but this caused a very weird message when the handles were used. This corrects them so they're in sync.
2. It documents a requirement for users of createPipe in that since we don't know who's going to use which part of the handles that it's the caller's responsibility to associate the handles with the correct I/O manager. This documents an example on how to do so.
3. Pipes can be opened in two modes, message and byte, essentially chunk and streaming mode. CreateNamedPipeW defaults to message mode while CreateFile defaulted to byte mode. However the modes need to match. This corrects it by always setting both modes explicitly ensuring that both ends always match. For these pipes we default to chunk instead of streaming mode as things such as iserv require the entire message in order to continue anyway.

This fixes the last issues to enable the new I/O manager with iserv support in GHC.

/cc @bgamari