Name cleanup and critical size reporting fix

I've fixed a critical bug that caused sizes to report 1024 times smaller. The `df` command returns 1k block counts, but New Relic showed that as byte counts.

Also cleaned up a bit the naming to feel more Linux-like and be more consistent across the code and scripts.

Plus a dedicated Debian/Ubuntu script using start-stop-daemon which doesn't require the newrelic user to have a shell and with /etc/default support to make changing the user and path easier.