Deduplicate files in /usr/share/doc using hardlinks.

Run this as late as possible to avoid the unwanted packages removal step to
later delete documentation that is shared with packages we want to keep.

Ignore timestamps in the process.

config/chroot_local-hooks/99-dedup_usr_share_doc

+#!/bin/sh
+
+/usr/bin/hardlink -t /usr/share/doc
+apt-get --yes purge hardlink