tty: Make termios and winsize being optional params

The dangling slave peers might have no data associate
with them if master peer is closed and link is hanging
up. Thus make this parameters optional to not blow the
image with data which never will be used.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>