Change. socket:setfd() set fd to invalid value.

Change. socket:setfd return previews `fd` value.
Change. socket.tcp and socket.udp can create object based on `fd`.

Add. test setfd
```lua
  ---- server
  cli = assert(srv:accept())
  -- test if worker thread take ownership and clear `fd`
  if run_worker_thread(cli:getfd()) then cli:setfd() end
  cli:close()

  ---- worker
  -- wrap raw `fd` to socket object
  local sock = socket.tcp(..., "client")
  -- do work with sock object
```

src/tcp.c

@@ -171,8 +171,11 @@ static int meth_getfd(lua_State *L)
 static int meth_setfd(lua_State *L)
 {
     p_tcp tcp = (p_tcp) auxiliar_checkgroup(L, "tcp{any}", 1);
-    tcp->sock = (t_socket) luaL_checknumber(L, 2);
-    return 0;
+    t_socket fd = tcp->sock;
+    if(lua_gettop(L) == 1) tcp->sock = SOCKET_INVALID;
+    else tcp->sock = (t_socket) luaL_checknumber(L, 2);
+    lua_pushnumber(L, (int) fd);
+    return 1;
 }
 
 static int meth_dirty(lua_State *L)
@@ -356,14 +359,24 @@ static int meth_settimeout(lua_State *L)
 \*-------------------------------------------------------------------------*/
 static int tcp_create(lua_State *L, int family) {
     t_socket sock;
-    const char *err = inet_trycreate(&sock, family, SOCK_STREAM);
+    const char *err;
+    const int is_inherite = lua_isnumber(L,1);
+    const char *obj_type = "tcp{master}";
+    if (is_inherite){
+        err = NULL;
+        sock = (t_socket)lua_tonumber(L, 1);
+        if(strstr(luaL_optstring(L,2,""),"client"))
+            obj_type = "tcp{client}";
+    }
+    else 
+        err = inet_trycreate(&sock, family, SOCK_STREAM);
     /* try to allocate a system socket */
     if (!err) {
         /* allocate tcp object */
         p_tcp tcp = (p_tcp) lua_newuserdata(L, sizeof(t_tcp));
         memset(tcp, 0, sizeof(t_tcp));
         /* set its type as master object */
-        auxiliar_setclass(L, "tcp{master}", -1);
+        auxiliar_setclass(L, obj_type, -1);
         /* initialize remaining structure fields */
         socket_setnonblocking(&sock);
         if (family == PF_INET6) {

src/udp.c

@@ -284,8 +284,11 @@ static int meth_getfd(lua_State *L) {
 /* this is very dangerous, but can be handy for those that are brave enough */
 static int meth_setfd(lua_State *L) {
     p_udp udp = (p_udp) auxiliar_checkgroup(L, "udp{any}", 1);
-    udp->sock = (t_socket) luaL_checknumber(L, 2);
-    return 0;
+    t_socket fd = udp->sock;
+    if(lua_gettop(L) == 1) udp->sock = SOCKET_INVALID;
+    else udp->sock = (t_socket) luaL_checknumber(L, 2);
+    lua_pushnumber(L, (int) fd);
+    return 1;
 }
 
 static int meth_dirty(lua_State *L) {
@@ -408,7 +411,14 @@ static int meth_setsockname(lua_State *L) {
 \*-------------------------------------------------------------------------*/
 static int udp_create(lua_State *L, int family) {
     t_socket sock;
-    const char *err = inet_trycreate(&sock, family, SOCK_DGRAM);
+    const char *err;
+    const int is_inherite = lua_isnumber(L,1);
+    if (is_inherite){
+        err = NULL;
+        sock = (t_socket)lua_tonumber(L, 1);
+    }
+    else 
+        err = inet_trycreate(&sock, family, SOCK_DGRAM);
     /* try to allocate a system socket */
     if (!err) {
         /* allocate udp object */

test/test_setfd.lua

@@ -0,0 +1,30 @@
+local socket = require "socket"
+
+local host, port = "127.0.0.1", "5462"
+
+local srv = assert(socket.bind(host, port))
+
+local sock_cli = socket.tcp()
+assert(sock_cli:connect(host, port))
+
+local fd do
+  local sock_srv = assert(srv:accept())
+  fd = assert(sock_srv:getfd())
+
+  assert(not pcall(function() sock_srv:setfd(nil) end))
+  sock_srv:setfd()
+  assert(sock_srv:close())
+end
+
+local sock_srv = assert(socket.tcp(fd, "client"))
+collectgarbage"collect"
+collectgarbage"collect"
+
+assert(5 == assert(sock_srv:send("hello")))
+assert("hello" == assert(sock_cli:receive(5)))
+
+sock_cli:close()
+sock_srv:close()
+srv:close()
+
+print("done!")