Upstart job: respawn cjdns if it crashes, but no more than once in 180 seconds

I've learned the hard way that cjdns is not entirely stable yet as far as long uptimes go. To prevent important nodes from going out randomly, I'm adding auto-respawning cjdns in case it crashes to Upstart job. 

cjdns will be respawned unless it exits with exit code 0 or is stopped via the Upstart job, but no more than one in 180 seconds to avoid infinite loops on startup with e.g. invalid config file or other error conditions.