WIP: software/dufs: handle certificate renewal

Because dufs only reads certificate on startup it does not detect when the certificate was renewed.

The only thing supported by dufs is to restart, but we can not restart the service, because the partition can not communicate with the supervisor. To solve this, use another level of process supervisor: using zdaemon we can run a process in the foreground, so that it stays under slapos' supervisor control, but still expose a way to restart the process using zdaemon API.