util/mkerr.pl: allow module names prefixed with OSSL_ or OPENSSL_

To make sure that our symbols don't clash with other libraries, we
claim the namespaces OSSL and OPENSSL.  Because C doesn't provide
namespaces, the only solution is to have them as prefixes on symbols,
thus we allow OSSL_ and OPENSSL_ as prefixes.

These namespace prefixes are optional for the foreseeable future, and
will only be used for new modules as needed on a case by case basis,
until further notice.

For extra safety, there's an added requirement that module names -
apart from the namespace prefix - be at least 2 characters long.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3781)