Don't use daemon(1) in debathena-metrics

[broder]

I used to think that daemon(1) was awesome, and way cooler than something more standard like start-stop-daemon. Having actually spent some time looking through its codebase, I'm way less convinced.

(In particular, daemon(1)'s options to manipulate the environment cause it to set variables twice in the child's environment, which is what led to Anders and I discovering CVE-2010-1646)

We should change the debathena-metrics startup and shutdown code to use start-stop-daemon, or teach the metrics gatherer to daemonize itself.

[jdreed]

python-daemon is a thing, at least in Precise. The documentation is kind of terrible, but it does everything we need.