Add --all flag to kill

This allows a user to send a signal to all the processes in the
container within a single atomic action to avoid new processes being
forked off before the signal can be sent.

This is basically taking functionality that we already use being
`delete` and exposing it ok the `kill` command by adding a flag.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

libcontainer/process_linux.go

@@ -379,7 +379,7 @@ func (p *initProcess) wait() (*os.ProcessState, error) {
 	}
 	// we should kill all processes in cgroup when init is died if we use host PID namespace
 	if p.sharePidns {
-		killCgroupProcesses(p.manager)
+		signalAllProcesses(p.manager, syscall.SIGKILL)
 	}
 	return p.cmd.ProcessState, nil
 }