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
```

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!")