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

[Dietmar Maurer]

> 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.

What is the problem with that? Can't we use signalfd for a clean implementation?

The init.d script only known the start parameter of a sheep, so it is unclear to me how to get the node id?

Also, debian uses 'start-stop-daemon' to start and stop daemons. And that command send signals to stop
a daemon (see 'man start-stop-daemon')

- Dietmar