Dpkg::Changelog: Use Dpkg::Fields for field handling

Dpkg::Changelog: Use Dpkg::Fields for field handling

* scripts/Dpkg/Changelog.pm: Replace all field hashes
with Dpkg::Changelog::Entry objects.
(Dpkg::Changelog::Entry): Base on Dpkg::Fields::Object.
(data2rfc822): Use Dpkg::Fields::Object->output and fix
handling of user-defined fields.
(data2rfc822_mult): Merge into data2rfc822 (autodetect
whether the argument is an object or an array of object).
* scripts/Dpkg/Changelog/Debian.pm: Adapt for
Dpkg::Changelog::Entry changes.
* scripts/t/600_Dpkg_Changelog.t: Likewise.