Dpkg::Fields::Object: Support filehandles and strings for output and dump

Dpkg::Fields::Object: Support filehandles and strings for output and dump

* scripts/Dpkg/Fields.pm (dump): Allow to omit the
filehandle argument. If the function is called in
non-void context, also remove the printed string
to the caller. Together this avoids having to fiddle
with filehandles if the caller doesn't want to.
(output): Likewise.
* scripts/Dpkg/Changelog.pm (data2rfc822): Simplify
using this new behaviour.