* apt-pkg/deb/debindexfile.cc,

* apt-pkg/deb/debindexfile.cc,
  apt-pkg/deb/deblistparser.cc:
  - use OpenMaybeClearSignedFile to be free from detecting and
    skipping clearsigning metadata in dsc and Release files

We can't write a "clean" file to disk as not all acquire methods copy
Release files before checking them (e.g. cdrom), so this reverts recombining,
but uses the method we use for dsc files also in the two places we
deal with Release files