Fix dual stack mode issue for tcp socket with IPv6

[repeatedly]

See fluentd issue for the detail: fluent/fluentd#2682
TCPServer.new doesn't set IPV6_V6ONLY and no parameter for IPV6_V6ONLY.
Socket.tcp_server_sockets set IPV6_V6ONLY internally, so use it instead.

Test echo server sciprt for checking the behaviour.

• TCPServer

require "socket"

ths = []
['0.0.0.0', '::'].each { |host|
  ths << Thread.new {
    gs = TCPServer.new(host, 2300)

    while true
      Thread.start(gs.accept) do |s|
        p s
        print(s, " is accepted in #{host}\n")
        while s.gets
          s.write($_)
        end
        print(s, " is gone\n")
        s.close
      end
    end
  }
}

ths.each { |th| th.join }

• Socket.tcp_server_sockets

require "socket"

ths = []
['0.0.0.0', '::'].each { |host|
  ths << Thread.new {
    gs = Socket.tcp_server_sockets(host, 2300).first
    gs.autoclose = false
    gs = TCPServer.for_fd(gs.fileno)

    while true
      Thread.start(gs.accept) do |s|
        p s
        print(s, " is accepted in #{host}\n")
        while s.gets
          s.write($_)
        end
        print(s, " is gone\n")
        s.close
      end
    end
  }
}

ths.each { |th| th.join }

• tcp client

require 'socket'

['0.0.0.0', '::'].each { |host|
  socket = TCPSocket.open(host, 2300)
  socket.print("hello\n")
  socket.flush
  puts response = socket.gets
  socket.close
}

[repeatedly]

Tanaka-san suggests using Addrinfo class is more better. I modified the patch to use it.

https://twitter.com/tanaka_akr/status/1194870142305329153 (in japanese)

• Test script with Addrinfo.

require "socket"

ths = []
['0.0.0.0', '::'].each { |host|
  ths << Thread.new {
    gs = Addrinfo.tcp(host, 2300).listen(1024)
    gs.autoclose = false
    gs = TCPServer.for_fd(gs.fileno)
    printf("server is on %s\n", [host, '2300'].join(":"))

    while true
      Thread.start(gs.accept) do |s|
        p s
        print(s, " is accepted in #{host}\n")
        while s.gets
          s.write($_)
        end
        print(s, " is gone\n")
        s.close
      end
    end
  }
}

ths.each { |th| th.join }

[repeatedly]

@nurse @ganmacs If no problem, I will merge and release new version.