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.