integrate Anthonys rred with POC for client-side merge

integrate Anthonys rred with POC for client-side merge

Providing the benefits of both without the downsides :)
(ABI breaks or external dependencies)

For this Anthonys rred is equipped with:
- magic-filename-pickup of patches rather than explicit messages
- use of FileFd instead of FILE* to get on-the-fly uncompress
  of the gzip compressed pdiff patches

The acquire code in turn stops checking for apt-file's helper
as our own rred is now clever enough for our needs.