(copyright-update-year): New function extracted from copyright-update.

(copyright-update-year): New function extracted from copyright-update.
When `arg' is set, replace the year, not the `copyright' text.
Ignore `copyright-update = nil' if called interactively.
Use "," rather than ", " if that's what was used before.
Recognize mixes of 2 and 4 digit years.
Use replace-match.
(copyright-update): Add interactivep arg.  Use it instead of use last-command.
Use replace-match.  Don't assume (point-min) == 1.