Pass an integer to stream_select() for correctness

Summary: Ref T4627. Passing `$timeout_sec` is slightly more correct here, although there is no change to program behavior.

Test Plan:
Used this script to verify that passing `0` and `0.999321` (for example) have the same effect to `stream_select()`:

  <?php

  $spec = array(
    0 => array('pipe', 'r'),
    1 => array('pipe', 'w'),
    2 => array('pipe', 'w'),
  );

  $pipes = array();
  $proc = proc_open('sleep 30', $spec, $pipes);
  list($stdin, $stdout, $stderr) = $pipes;

  $read = array($stdout, $stderr);
  $write = array();
  $except = array();

  $select_start = microtime(true);
  stream_select($read, $write, $except, 0.999321, 80000);
  $select_end = microtime(true);

  printf(
    "select() took %d us to return.\n",
    ($select_end - $select_start) * 1000000);

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4627

Differential Revision: https://secure.phabricator.com/D8567