fix systemd for Ubuntu 14.10

It has systemctl installed on the system but does not have systemd
wired up as init by default and trying to use systemctl throws
exceptions that breaks everything.

Also add a big banner warning because I'm sure someone is going to
think that the shortest past to fixing priority bugs is hacking up
what gets reported from these modules, rather than fixing the bugs
in the downstream logic.

closes #2332
Similarly this can be `@systemctl_path ||= which("systemctl")`