MEDIUM: ssl: emulate multi-cert bundles loading in standard loading

MEDIUM: ssl: emulate multi-cert bundles loading in standard loading

Like the previous commit, this one emulates the bundling by loading each
certificate separately and storing it in a separate SSL_CTX.
This patch does it for the standard certificate loading, which means
outside directories or crt-list.

The multi-certificates bundle was the common way of offering multiple
certificates of different types (ecdsa and rsa) for a same SSL_CTX.
This was implemented with OpenSSL 1.0.2 before the client_hello callback
was available.

Now that all versions which does not support this callback are
deprecated (< 1.1.0), we can safely removes the support for the bundle
which was inconvenient and complexify too much the code.