ALPN is a TLS extension for application-layer protocol negotiation
within the TLS handshake [1].
This patch extends the Listener API to include a new 'alpn_protocols'
parameter. With this parameter, users can set an ALPN preference list
(descending order of preference).
Presently, the amphora provider driver is limited to http/1.0 and
http/1.1 ALPN protocol IDs. Support for "h2" (HTTP/2 over TLS) depends
on HAProxy 2.0 or newer.
[1] https://tools.ietf.org/html/rfc7301
Change-Id: If08a8169498cdfaa75440e8971ba0caff45ac4c4
a5f0524fd0