Allow fd zero as return value from socket

[0xJonas]

The following code incorrectly errors:

#lang racket

(require racket/unix-socket)
(close-input-port (current-input-port))
(unix-socket-listen "socket")

$ racket test.rkt 
unix-socket-listen: failed to create socket
  errno: 2
  error: No such file or directory
  context...:
   /usr/local/share/racket/pkgs/unix-socket-lib/racket/unix-socket.rkt:226:3
   /usr/local/share/racket/collects/ffi/unsafe/atomic.rkt:73:13
   /usr/local/share/racket/collects/racket/contract/private/arrow-val-first.rkt:489:18
   body of "/home/delphy4096/src/racket-unix-socket/test.rkt"

Closing stdin makes file descriptor 0 available to be used by socket, but the check with positive? only allows return values greater than 0. This makes the function raise an error, even though there is none.

[rmculpepper]

Thanks!