Don't include devicenodes in piuparts system.tar.xz

piuparts (in unstable) can't handle device nodes and fails on a tar with them when run as a normal user. They are also not needed and can be removed with: xzcat system.tar.xz | mmtarfilter --path-exclude='/dev/*' | xz > system2.tar.xz. But it would be easier if debusine would not include them. For mmdebstrap this can be done with --skip=output/dev.