SSL.session: Refactored handling of signature keys.

The local (typically server-side) key is now session->private_key,
and the remote (typically client-side) validation key is now
session->peer_public_key. These two variables replace the now
removed session->rsa, session->dsa and session->ecdsa.

This change is needed to avoid confusion when also having client
certificates which need validation.

Preparation for supporting selecting server certificate based on
multiple criteria.
SSL.handshake: Added a note about TLS vs SSLv2 compat.
Fixed warning.
Fixed warnings.
SSL.state: Minor semantic change in alert handling.
Mysql: Avoid pulling in OpenSSL if not needed.
Mysql: Fixed disabling of MariaDB.

There were some typos in the tests of ${MARIADB_CONFIG}.
SSL.context: Unified the certificate keys.

As only one of context->rsa, context->dsa and context->ecdsa
can be active at a time, unify them into a single variable.