Previously, only one client packet could be processed in a udp stream session
even though multiple response packets were supported.  Now multiple packets
coming from the same client address and port are delivered to the same stream
session.

If it's required to maintain a single stream of data, nginx should be
configured in a way that all packets from a client are delivered to the same
worker.  On Linux and DragonFly BSD the "reuseport" parameter should be
specified for this.  Other systems do not currently provide appropriate
mechanisms.  For these systems a single stream of udp packets is only
guaranteed in single-worker configurations.

The proxy_response directive now specifies how many packets are expected in
response to a single client packet.

Cast to intermediate "void *" to lose compiler knowledge about the original
type and pass the warning.  This is not a real fix but rather a workaround.

Found by gcc8.

In mail and stream modules, no certificate provided is a fatal condition,
much like with the "ssl" and "starttls" directives.

In http, "listen ... ssl" can be used in a non-default server without
certificates as long as there is a certificate in the default one, so
missing certificate is only fatal for default servers.

Now it's clear from log error message if the error occurred on client or
upstream side.

The variable keeps a comma-separated list of protocol names from ALPN TLS
extension defined by RFC 7301.

No functional changes.

Based on patches by Johannes Baiter <johannes.baiter@bsb-muenchen.de>
and Calin Don.