gromox/{imap,pop3} don't support RSA and ECDSA certificates in parallel

[robert-scheck]

As of writing, gromox/{imap,pop3} don't seem to support RSA- and ECDSA-based SSL certificates in parallel. According to the code in mra/imap/imap_parser.cpp:154 SSL_CTX_use_PrivateKey_file() is used, which only handles the first private key. The expectation is however to have both certificate types in parallel supported for hybrid setups, to cover legacy and modern clients. This equals at NGINX to the repeated usage of ssl_certificate and ssl_certificate_key (once for RSA, once for ECDSA). Any chance to have this in the future?

Technical Guideline TR-02102-2: Cryptographic Mechanisms: Recommendations and Key Lengths from the German Federal Office for Information Security ("BSI") recommends ECDSA-based ciphers, which unfortunately can currently not be configured in gromox/{imap,pop3}.