* ftparchive/writer.cc:

* ftparchive/writer.cc:
  - write LongDescriptions if they shouldn't be included in Packages
    file into i18n/Translation-en by default.

It is ensured that each package+description is listed only ones in the
Translation file even if we generate multiple Packages file in one run.

The file is only generated in "generate" - the simple file commands
can't create it by now. Also, the LongDescription is currently a
global setting, so generating archives with and without LongDescriptions
in the Packages file in the same run are currently not possible.