maint: clean up update-b4-copyright code

* build-aux/update-b4-copyright: Do not accept 2-digit
UPDATE_COPYRIGHT_YEAR, which was not handled correctly.
Don't accept a `[' in a b4_copyright argument.
Format code more consistently.
Don't assume b4*copyright never occurs.
(cherry picked from commit 0b61a8ec1842bfbd6130714d06b758165b32ead4)