Support ALPN.

This change adds support for ALPN[1] in OpenSSL. ALPN is the IETF
blessed version of NPN and we'll be supporting both ALPN and NPN for
some time yet.

Cherry-picked from 6f017a8f.

[1] https://tools.ietf.org/html/draft-ietf-tls-applayerprotoneg-00