Try any conceivable way to detect a Linux Mint system of any kind

This additions to the debian inventory try to detect a Linux Mint system by checking /etc/lsb-release, /etc/os-release, /etc/linuxmint/info (overlapping with lsb.cf can be safely removed if it makes sense). At the end of the process, classes like linuxmint, lmde, linuxmint_14, nadia may be defined.