Add Mageia support

Add Mageia support

This patch adds support for the Mageia Linux distribution:
 http://www.mageia.org/

Mageia is a fork of Mandriva although some divergence has already occured
and thus inclusion of these changes upstream allow us to (hopefully)
migrate more rapidly to the new standard approaches systemd offers.
Indeed, we already use the preferred mechanism of OS identification via
the /etc/os-release file rather than a distro specific variation.

This patch mostly mirrors the patch added previously for Mandriva
support. In addition to those original authors, this patch was mostly
written by Dexter Morgan with help from Colin Guthrie and Eugeni Dodonov.