Fix minor Y10k bugs.

Fix minor Y10k bugs.

* lisp/arc-mode.el (archive-unixdate):
* lisp/autoinsert.el (auto-insert-alist):
* lisp/calc/calc-forms.el (math-this-year):
* lisp/gnus/nnweb.el (nnweb-google-parse-1):
* lisp/emacs-lisp/copyright.el (copyright-current-year)
(copyright-update-year, copyright):
* lisp/tar-mode.el (tar-clip-time-string):
* lisp/time.el (display-time-update):
Don't assume years have 4 digits.