tty: Make sure no /dev/tty inheritance exist

Currently we support restoring opened /dev/tty reference
if only control terminal belongs to the same process,
ie no inheritance is allowed.

Thus we should refuse to dump in such scenario
otherwise restore will fail.

Reported-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>