Read siginfo-s from images and send them to itself by sigqueueinfo.

Thread signals cannot be restored in restore_thread_common, because
it blocks SIGCHLD, which used for error detecting.

v2: Don't remap task_args and thread_args
v3: fix error handling
v4: cosmetic clean up
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>