Add YAML output format for rozo agent status

if 'rozo agent' listens to components:
>   $ rozo agent status
>   listeners: [storaged, exportd, rozofsmount]
>   pid: '12587'
otherwise:
>   rozo agent status
>   rozo: no agent is running.
wrong fix to bug #52 ... pid of 'rozo agent' is readable for any user
[rozo] add volume output