openssl: add ALPN support

Add ALPN support when using OpenSSL. This will offer ALPN and NPN to the
server, who can respond with either one or none of the two. OpenSSL >=
1.0.2 is required, which means as of today obtaining a snapshot from
ftp://ftp.openssl.org/snapshot/.

See:
http://tools.ietf.org/html/draft-ietf-tls-applayerprotoneg-04
https://github.com/openssl/openssl/blob/ba168244a14bbd056e502d7daa04cae4aabe9d0d/ssl/ssl_lib.c#L1787