This can be useful to understand why "no live upstreams" happens,
in particular.

When configured, an individual listen socket on a given address is
created for each worker process.  This allows to reduce in-kernel lock
contention on configurations with high accept rates, resulting in better
performance.  As of now it works on Linux and DragonFly BSD.

Note that on Linux incoming connection requests are currently tied up
to a specific listen socket, and if some sockets are closed, connection
requests will be reset, see https://lwn.net/Articles/542629/.  With
nginx, this may happen if the number of worker processes is reduced.
There is no such problem on DragonFly BSD.

Based on previous work by Sepherosa Ziehau and Yingqi Lu.

There is no need to set "i" to 0, as it's expected to be 0 assuming
the bindings are properly sorted, and we already rely on this when
explicitly set hport->naddrs to 1.  Remaining conditional code is
replaced with identical "hport->naddrs = i + 1".

Identical modifications are done in the mail and stream modules,
in the ngx_mail_optimize_servers() and ngx_stream_optimize_servers()
functions, respectively.

No functional changes.

The variable keeps time spent on establishing a connection with
the upstream server.

The http and stream versions of this macro were identical.

It's now enough to specify proxy_protocol option in one listen directive to
enable it in all servers listening on the same address/port.  Previously,
the setting from the first directive was always used.