Cygwin: pty: Fix script command crash on console in Win7.

Cygwin: pty: Fix script command crash on console in Win7.

- Previously, the command "cmd /c script -c cmd" in console of Win7
  crashes. This seems to be due to a bug (?) of AttachConsole().
  This patch adds workaround for this issue.

  Currently, pty reattaches to the console of the process which is
  predetermined by ConsoleProcessList() after temporarily attaching
  to another console. After that, the console output handle opened
  with the name "CONOUT$" may not be accessible in Win7.
  This seems to happen when the attached process does not have the
  same handle even if the console attached is the same. With this
  patch, cygwin-console-helper which is started when pty master is
  opened in console, is utilized to be a target process to which
  pty reattaches if the OS is Win7.