Cygwin: pty, termios: Unify thoughts of read ahead beffer handling.

Cygwin: pty, termios: Unify thoughts of read ahead beffer handling.

- Return value of eat_readahead() is redefined. The return values
  of fhandler_termios::eat_readahead() and fhandler_pty_slave::
  eat_readahead() were little bit different. This patch unifies
  them to number of bytes eaten by eat_readahead().
- Considerration for raixget() is added to fhandler_pty_master::
  accept_input() code.
- Transfering contents of read ahead buffer in
  fhandler_pty_master::transfer_input_to_pcon() is removed since
  it is not necessary.
- fhandler_pty_slave::eat_readahead() ckecks EOL only when ICANON
  is set.
- Guard for _POSIX_VDISABLE is added in checking EOL.