afserver: avoid requiring a specific SSL protocol version

Trust the clients to negotiate the latest/best protocol version they can.

This should generally improve security over time (as OpenSSL improves
and implements improved protocols) without having the APF codebase
or server/client deployments need to chase latest OpenSSL API additions,
causing portability problems, or breaking client deployments that
are hard to upgrade.
afclient: use whatever version of TLS (or better) we can.

Refuse to use pre-TLS SSL, since now SSLv3 has been broken by POODLE attack.