Append run output to a log and list leftover .new configs

Non-interactive runs leave obsolete .new config files in place: new-config
exits without acting and there was no record to debug later. Tee the whole
run (script messages plus slackpkg output) to LOG, default
/var/log/dist-upgrade.log, with a timestamped header per run (a second one
on a 50-restart). LOG= disables it.

Also report leftover .new files under /etc and /boot at the end of a run.

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