tty: Implement support of current tty

Opening current tty is tricky: first slave peer should
be opened and session restored, and only then we can
open /dev/tty. So that I made rst_info to carry
additional list @tty_ctty where all current ttys
get gathered and opened after slave peers were
brought to live.

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