Add option to set dpkg file suffix, to avoid file name conflicts for buildinfo and changes files

Add an option --dpkg-file-suffix to add a suffix to the filename of the changes and buildinfo files generated by dpkg.

This could be set on the buildds to work around issues with duplicate buildinfo filenames like https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=869184

This also avoid the issue where the maintainer uploads a _amd64.changes file, and the buildd uses the same name. This happens when maintainers manually remove the binaries to do a source-only upload. This would potentially also happen if dak supported throw-away binaries.

The dpkg options used aren't supported on jessie (and older). The patch checks the version of dpkg-dev in the build environment. If this is too old, the option is ignored.