Call kill command using an absolute path. Also, update syntax and status codes.

When stopping workers, the ```kill``` command syntax issued to ```exec()``` is invalid, so an error is returned from ```exec()```

```sh
Killing 31284 ... kill: 1: Illegal number: SIGQUIT
```

and the processes are not killed, but the workers are unregistered, so they can't be seen anymore.

From kill manual page:
Your shell (command line interpreter) may have a built-in kill command.  You may need  to  run  the  command  described  here  as /bin/kill to solve the conflict.