fix logic for generating DEBIAN/conffiles
Two errors fixed here.. 1. where we had an extant DEBIAN/conffiles, we performed a 'find' and attempted to grep out line already in that file. However, the grep was non-anchored so risked having false hits from substrings in a larger string 2. conffiles should "be listed as absolute pathnames", but we were omitting the leading slash. Rather than try to grep, let's just generate our list of conffiles and make sure it's been through "sort -u". I've checked that switching from a list of conffiles without leading slashes to having them be present does not cause problems when upgrading. For example, unattended-upgrades says.. found pkg: chem-zfs-backup-server conffile line: /etc/init.d/zfs-backup 3dabfa1996c2405aae81b4468f6cfb6f current md5: 3dabfa1996c2405aae81b4468f6cfb6f and proceeds unimpeded. And, an interactive "apt-get install" does not lead to any conffile-related prompts.