Support chkconfig properly

Extra commit to find systemctl correctly and to include `paths.cf` (it was already required, this is a formality).

This commit implements the `disable` policy for `chkconfig`-capable systems.  See examples for the extremely simple usage.

The only remarkable thing here is that we have `services` promises calling a `service_method` which calls an agent bundle, which calls `services` promises, and so on.  It's extremely cool but may confuse inexperienced users.