Import IPv6-supported Lua5.2 compatible code from Matthew Wild's repo

Signed-off-by: Vadim A. Misbakh-Soloviov <mva@mva.name>

samples/echoclnt.lua

@@ -0,0 +1,24 @@
+-----------------------------------------------------------------------------
+-- UDP sample: echo protocol client
+-- LuaSocket sample files
+-- Author: Diego Nehab
+-- RCS ID: $Id: echoclnt.lua,v 1.10 2005/01/02 22:44:00 diego Exp $
+-----------------------------------------------------------------------------
+local socket = require("socket")
+host = host or "localhost"
+port = port or 7
+if arg then
+    host = arg[1] or host
+    port = arg[2] or port
+end
+host = socket.dns.toip(host)
+udp = assert(socket.udp())
+assert(udp:setpeername(host, port))
+print("Using remote host '" ..host.. "' and port " .. port .. "...")
+while 1 do
+	line = io.read()
+	if not line or line == "" then os.exit() end
+	assert(udp:send(line))
+	dgram = assert(udp:receive())
+	print(dgram)
+end