[sheepdog] killing sheep is unsafe - sheep: add option to create pid file

[Jens WEBER]

> 
> I am thinking of adding a new collie command,
> 
>  collie node kill node_id
> 
> can this meet the needs to shutdown the specific node? I don't want to
> implement a signal handler for TERM, because we don't want worker
> threads to handle signals as normal.
> 
> Thanks,
> Yuan
> 
That would be nice for maintenance too. Wiki mention SD_OP_KILL_NODE but seems not implemented yet.

Thanks Jens