dump: Reseve space and construc sigframes for parasites

We're about to implement functionality to allow parasite to
return to dumpee via sigreturn in case crtools suddenly detached
from it. Thus, we need a space for sigframe in shared area.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>