tty: notify about orphan tty-s via rpc

Now Docker creates a pty pair from a container devpts to use is as console.
A slave tty is set as a control tty for the init process and bind-mounted
into /dev/console. The master tty is handled externelly.

Now CRIU can handle external resources, but here we have internal resources
which are used externaly.

https://github.com/opencontainers/runc/issues/1202


travis-ci: success for A few fixes to c/r a docker container with a console (rev3)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>