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/state_linux.go

@@ -39,7 +39,7 @@ type containerState interface {
 
 func destroy(c *linuxContainer) error {
 	if !c.config.Namespaces.Contains(configs.NEWPID) {
-		if err := killCgroupProcesses(c.cgroupManager); err != nil {
+		if err := signalAllProcesses(c.cgroupManager, syscall.SIGKILL); err != nil {
 			logrus.Warn(err)
 		}
 	}