With this change, it is now possible to use ngx_conf_merge_ptr_value()
to merge keyval arrays.  This change actually follows much earlier
changes in ngx_conf_merge_ptr_value() and ngx_conf_set_str_array_slot()
in 1452:cd586e963db0 (0.6.10) and 1701:40d004d95d88 (0.6.22).

To preserve compatibility with existing 3rd party modules, both NULL
and NGX_CONF_UNSET_PTR are accepted for now.

Reworked connections reuse, so closing connections is attempted in
advance, as long as number of free connections is less than 1/16 of
worker connections configured.  This ensures that new connections can
be handled even if closing a reusable connection requires some time,
for example, for a lingering close (ticket #2017).

The 1/16 ratio is selected to be smaller than 1/8 used for disabling
accept when working with accept mutex, so nginx will try to balance
new connections to different workers first, and will start reusing
connections only if this won't help.

Previously, reusing connections happened silently and was only
visible in monitoring systems.  This was shown to be not very user-friendly,
and administrators often didn't realize there were too few connections
available to withstand the load, and configured timeouts (keepalive_timeout
and http2_idle_timeout) were effectively reduced to keep things running.

To provide at least some information about this, a warning is now logged
(at most once per second, to avoid flooding the logs).

Reported by Jinhua Tan.

When changing binary, sending a SIGTERM to the new binary's master process
should not remove inherited UNIX sockets unless the old binary's master
process has exited.