Enhance util/openssl-update-copyright shell script

- Avoid creating repeated year ranges <current_year>-<current_year> for
  documents which were added in the current year.

- Use extended regular expressions for better readability (less quoting)

- Use a shebang line which is more portable

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5044)