Set TCP listen backlog for API socket to SOMAXCONN

A value of 1 is too low for heavy users of the API, such as the weldr-web
interface.

This is also systemd's default for sockets it opens. Using lorax-composer with
socket activation already results in a backlog of SOMAXCONN connections.

(cherry picked from commit be5d50e6f3)

Related: rhbz#1613058

src/sbin/lorax-composer

@@ -230,7 +230,7 @@ if __name__ == '__main__':
         listener.bind(opts.socket)
         os.chmod(opts.socket, 0o660)
         os.chown(opts.socket, 0, gid)
-        listener.listen(1)
+        listener.listen(socket.SOMAXCONN)
 
     start_queue_monitor(server.config["COMPOSER_CFG"], uid, gid)