copyright-current-year wrong after new year

[Jamie Lokier]

Dear Emacs authors,

I'm using Emacs' copyright.el to update copyright notices.

I left an Emacs running since 2000, and today, when saving a file that
already says "Copyright (C) 2000, 2001 Jamie Lokier" in it, Emacs asked
me whether I wanted to update the copyright year to 2000.

I said no, but if I'd said yes it would have changed the line to
"Copyright (C) 2000, 2001, 2000 Jamie Lokier".

This happened because `copyright-current-year' is a constant defined
when copyright.el is loaded.  The fix is to calculate that value on the
fly each time.

enjoy,
-- Jamie