Previously, listenings sockets were not cloned if the worker_processes
directive was specified after "listen ... reuseport".

This also simplifies upcoming configuration check on the number
of worker connections, as it needs to know the number of listening
sockets before cloning.

Errors when sending UDP datagrams can happen, e.g., when local IP address
changes (see fa0e093b64d7), or an unavailable DNS server on the LAN can cause
send() to fail with EHOSTDOWN on BSD systems.  If this happens during
initial query, retry sending immediately, to a different DNS server when
possible.  If this is not enough, allow normal resend to happen by ignoring
the return code of the second ngx_resolver_send_query() call, much like we
do in ngx_resolver_resend().

Before 4a8c9139e579, ngx_resolver_create() didn't use configuration
pool, and allocations were done using malloc().

In 016352c19049, when resolver gained support of several servers,
new allocations were done from the pool.

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.