Description:
------------
All eio_* functions check if $fd is less than or equal to zero. However, fd/0 is 
usually stdin, but it also can be any other file. There's no reason to prevent eio 
from operating on fd/0.

Test script:
---------------
<?php
echo eio_close(0) ? "closing stdin\n" : "failed to close stdin\n";
echo eio_close(2) ? "closing stderr\n" : "failed to close stderr\n";
eio_event_loop();


Expected result:
----------------
closing stdin
closing stderr


Actual result:
--------------
failed to close stdin
closing stderr