Stream: support ALPN for proxy_ssl upstream.

Stream: support ALPN for proxy_ssl upstream.

Added the proxy_ssl_alpn directive, which sets the list of protocols
to advertise via ALPN during upstream TLS handshakes.  Each argument
is a complex value, so variables are accepted.  In particular,

    proxy_ssl_alpn $ssl_alpn_protocol;

inherits the protocol negotiated in the downstream TLS handshake.

When all evaluated values are empty or absent, no ALPN extension is
sent, equivalent to the directive not being set at all.

Closes #406 on GitHub.