Cygwin: pty: Inherit typeahead data between two input pipes.

Cygwin: pty: Inherit typeahead data between two input pipes.

- PTY has a problem that the key input, which is typed during windows
  native app is running, disappears when it returns to shell. This is
  beacuse pty has two input pipes, one is for cygwin apps and the other
  one is for native windows apps. The key input during windows native
  program is running is sent to the second input pipe while cygwin
  shell reads input from the first input pipe. This issue had been
  fixed once by commit 29431fcb, however, the new implementation of
  pseudo console support by commit bb428520 could not inherit this
  feature. This patch realize transfering input data between these
  two pipes bidirectionally by utilizing cygwin-console-helper process.
  The helper process is launched prior to starting the non-cygwin app,
  however, exits immediately unlike previous implementation.