More fixups to make the new IO lib work on mingw.
* Outlaw changing the file position on a text-mode file.  After
  consideration of the mingw translation semantics I cannot see
  how to make a correct implementation.
* Add a field to Handle__ to say whether or not the handle is in
  binary mode.
* Restrict seek operations on Handles to those in binary mode.
* Export hSetBinaryMode from IO.lhs.