dump: Let task handle signals on seize

When seizing a task and waiting for it to stop, we can receive an event
from kernel, that the task didn't stop, but caught a signal. Don't treat
this as an error, let the victim handle it and proceed.

The transition/fork test should work OK now.

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