conf, router: Make the listen(2) backlog configurable

@oopsoop2 on GitHub reported a performance issue related to the default
listen(2) backlog size of 511 on nginx. They found that increasing it
helped, nginx has a config option to configure this.

They would like to be able to do the same on Unit (which also defaults
to 511 on some systems). This seems reasonable.

NOTE: On Linux before commit 97c15fa38 ("socket: Use a default listen
backlog of -1 on Linux") we defaulted to 511. Since that commit we
default to the Kernels default, which before 5.4 is 128 and after is
4096.

This adds a new per-listener 'backlog' config option, e.g

  {
      "listeners": {
          "[::1]:8080": {
              "pass": "routes",
              "backlog": 1024
          },
      }

      ...
  }

This doesn't effect the control socket.

Closes: https://github.com/nginx/unit/issues/1384
Reported-by: <https://github.com/oopsoop2>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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