cr: Support zombie tasks

Dump the core-pid.img file only. On restore select the way of killing
task based on his exit_code -- exit or kill with a signal. Before dying
unblock all the handlers and set SIG_DFL to it (to make the dead signal
other than KILL be deliverable).

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