MEDIUM: ssl: provide our own BIO.

MEDIUM: ssl: provide our own BIO.

Instead of letting the OpenSSL code handle the file descriptor directly,
provide a custom BIO, that will use the underlying XPRT to send/recv data.
This will let us implement QUIC later, and probably clean the upper layer,
if/when the SSL code provide its own subscribe code, so that the upper layers
won't have to care if we're still waiting for the handshake to complete or not.