TS-2239: detect OpenSSL ALPN support
TS-2239: support the ALPN TLS extension

Add support the for TLS Application Layer Protocol Negotiation
extension. This uses the same infrastructure as NPN, so a consistent
protocol advertisement is created. Any plugin that registers to
accept a named protocol will automatically gain NPN and ALPN support.

This requires a very recent OpenSSL, probably 1.2 or something like
that.
TS-2239: use full TLS extension names in the configure log
TS-2239: CHANGES