Safer implementation of request_input()/output() in CLI mode

As an implementation detail, prior releases used php://input and php://output in all modes.

This release changes to use php://stdin and php://stdout instead, as php://input and php://output are not "real" FDs, which can lead to observable differences, e.g.:

<?php
$f = fopen('php://stdin', 'r');
stream_set_blocking($f, false);
fgets($f);
var_dump(feof($f)); // false for php://stdin, true for php://input