generate fullchain cert

[prabirshrestha]

Fixes https://github.com/JessThrysoee/synology-letsencrypt/issues/22

This generates fullchain cert in $cert_path so that it can be used in traefik. While $archive_path contains the fullchain.pem it is hard to locate this directory as it contains id and can be hard to use in dockers compose. Having fullchain cert generate in $cert_path means I can now have a easy to use path such as /usr/local/etc/synology-letsencrypt/example.com/certificates/example.com.fullchain.crt

[JessThrysoee]

Thanks for taking the time to create a pull request.

$cert_domain.crt contains the full chain, so you can use that.

This brought attention to the fact that the fullchain.crt generated erroneously contain $cert_domain.issuer.crt twice. Thanks.

See also #22