upgrades, janus: dist-upgrade fails to install new files

1. Need to implement force_upgrade for janus app.
2. Need to hold janus package during apt full-upgrade.
3. Even though it failed to replace the files (even unmodified) in /etc/janus, apt still considers janus as fully installed/configured at this point, and freedombox-dist-upgrade reports a success. (Although, with the above two changes, this won't happen for janus, but it points to a weakness in error handling.)

Aug 19 20:25:51 freedombox upgrades[216322]: pmstatus:dpkg-exec:76.9231:Running dpkg
Aug 19 20:25:51 freedombox upgrades[216322]: pmstatus:janus:76.9231:Preparing to configure janus (amd64)
Aug 19 20:25:51 freedombox upgrades[216322]: pmconffile:/etc/janus/janus.jcfg:76.9231:'/etc/janus/janus.jcfg' '/etc/janus/janus.jcfg.dpkg-new' 1 1
Aug 19 20:25:51 freedombox upgrades[216322]: pmerror:janus:76.9231:end of file on stdin at conffile prompt
Aug 19 20:25:51 freedombox upgrades[216322]: E: Sub-process /usr/bin/dpkg returned an error code (1)
Aug 19 20:25:51 freedombox upgrades[31152]: Running unattended-upgrade...
...
Aug 19 20:35:54 freedombox upgrades[31152]: Dist upgrade complete. Removing flag.