Make it easy to install extra binaries into subdirectories of extra-pkg. This makes it easier to manage the extra-pkg directory when it contains manually added packages and automatically added packages from one of the package tracks by putting the automatically added packages into extra-pkg/auto and manually added packages into extra-pkg/manual. To this end, add parameter subdir to PBuilder.add_binaries_to_extra_pkg method with default value "auto". Adapt the test case accordingly. Also add the command line argument --subdir to bin/treepkgbuilder.py add-binaries command, this time with default value "manual".