install apt preferences that prefer packages from mntre.com over the versions...

install apt preferences that prefer packages from mntre.com over the versions provided by Debian, making 'apt upgrade' safe

In !30 (closed) I talked about using a Protected:yes package to keep the package versions from mntre.com installed. Unfortunately, while this will successfully refuse to upgrade the packages in question to their unpatched variants, it also results in hard to interpret apt errors when apt is not able to find a solution. I think a better way is to use apt pinning. Thus, this merge request lets the reform-tools package ship a apt preferences configuration that will always prefer packages from mntre.com over all other sources while at the same time listing packages which must not be installed from Debian sources.