This option is used to mark external resources on dump.

Currently it's going to be used to handle external tty-s,
but in a future it can be used to any type of resources.

We can have a few ways to restore external resources and
we will have a separate options to say how to restore each type.

For example, we can use --inherit-fd to restore external
file descriptors.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>