Automate building packages for upstream trunk

  - Add debian/extra/build-upstream-master-dsc script. This checks
    out upstream git, applies debian/, applies some packaging modifications,
    applies most of our Debian patches (ignoring the backported upstream
    ones and ignoring fuzz), and builds a dsc.
 - Add debian/patches/upstream-master/: These patches override the ones in
   debian/patches/ for trunk builds, in order to apply.