tty, ioctls -- Add new ioctl definitions for tty flags fetching

This patch defines new ioctl codes TIOCGPKT, TIOCGPTLCK,
TIOCGEXCL for fetching pty's packet mode and locking state,
and exclusive mode of tty.

[ No real handlers for the codes though, this will be
  addressed in another patch for easier review and
  bisectability ]

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>