restore: restore sigaction for alive tasks

The helper task doesn't change sigaction and does nothing with
parent_sigacts. paren_sigacts will contain values for the previous alive
task, so the logic about inherence should work as expected.

Reported-by: Jenkins Criuovich
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>