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
```
