Reject SSL connection if ALPN is used but there's no common protocol

Reject SSL connection if ALPN is used but there's no common protocol

If the client supports ALPN but tries to use some other protocol, like
HTTPS, reject the connection in the server. That is surely a confusion
of some sort. Furthermore, the ALPN RFC 7301 says:

> In the event that the server supports no protocols that the client
> advertises, then the server SHALL respond with a fatal
> "no_application_protocol" alert.

This commit makes the server follow that advice.

In the client, specifically check for the OpenSSL error code for the
"no_application_protocol" alert. Otherwise you got a cryptic "SSL
error: SSL error code 167773280" error if you tried to connect to a
non-PostgreSQL server that rejects the connection with
"no_application_protocol". ERR_reason_error_string() returns NULL for
that code, which frankly seems like an OpenSSL bug to me, but we can
easily print a better message ourselves.

Reported-by: Jacob Champion
Discussion: https://www.postgresql.org/message-id/6aedcaa5-60f3-49af-a857-2c76ba55a1f3@iki.fi