Support chkconfig systems and disable service_policy

Extra commit to find systemctl correctly and to include paths.cf (it was already required, this is a formality).

This commit implements the `disable` `service_policy` for chkconfig- and systemd-capable systems.  I've tested chkconfig and need testing for systemd.

CFEngine doesn't have `enable` as a `service_policy` so I was wondering if I should always call `systemctl enable` when starting a service.  Not sure.