MINOR: ssl: add a get_alpn() method to ssl_sock

MINOR: ssl: add a get_alpn() method to ssl_sock

This is used to retrieve the TLS ALPN information from a connection. We
also support a fallback to NPN if ALPN doesn't find anything or is not
available on the existing implementation. It happens that depending on
the library version, either one or the other is available. NPN was
present in openssl 1.0.1 (very common) while ALPN is in 1.0.2 and onwards
(still uncommon at the time of writing). Clients are used to send either
one or the other to ensure a smooth transition.