The idea is to be able to add extension value lines directly on the
command line instead of through the config file, for example:

    openssl req -new -extension 'subjectAltName = DNS:dom.ain, DNS:oth.er' \
                     -extension 'certificatePolicies = 1.2.3.4'

Fixes #3311

Thank you Jacob Hoffman-Andrews for the inspiration
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4986)