debian: add `check_installed_by_apt` as an alias for `check_installed`
debian: correct the way enabled SysV services are detected

If a service is linked in `/etc/rc?.d`, it can be either disabled or
enabled. Enabled services are `S` links and disabled ones are `K` links.
debian: use dpkg-query to check package status and version

dpkg-query comes with dpkg and is available everywhere. It is a
preferred interface to extract fields from package control files.
Merge pull request #22 from vincentbernat/feature/debian

Debian related changes