crtools: dump pending signals (v4)

PTRACE_PEEKSIGINFO is used for received pending signals,
then all signal are sent back and saved in a image.

v2: rework according with the new kernel interface
v3: rework according with the newrest kernel interface
v4: fix error handling

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