Enable managing of /etc/apt/preferences

In the same way that Debian allows a "sources.list" in addition to entries in "sources.list.d," they allow a "preferences" in addition to entries in "preferences.d." This modification allows Puppet to manage the "preferences" file in the same way that it manages the "sources.list" file. Because of a bug in Debian (641245), "preferences" cannot be empty, or contain a comment on its last line, or "apt-get update" will succeed, but "apt-get upgrade" will fail.