Extend hConnectTo to also allow output handles to be connected, i.e.,

    h1 <- openFile "foo" WriteMode
    h2 <- openFile "bar" WriteMode
    hConnectTo h1 h2

  will cause h1's buffer to be flushed when h2's buffer overflows
  (and it is just about to be flushed.) The implementation is currently
  not as lazy as that, it flushes h1's buffer regardless of whether h2's
  buffer overflows or not.

  This is used to connect 'stderr' and 'stdout', i.e., output on
  'stderr' will now cause 'stdout' output to (first) be flushed.