Use php://input stream instead of building our own IPN validation array

Reroll. This makes sense since we just need to prefix the validate command.

Based on their example, there would technically be more to do to properly construct the array rather than reading from $_POST

// STEP 1: read POST data
// Reading POSTed data directly from $_POST causes serialization issues with array data in the POST.
// Instead, read raw POST data from the input stream.
$raw_post_data = file_get_contents('php://input');
$raw_post_array = explode('&', $raw_post_data);
$myPost = array();
foreach ($raw_post_array as $keyval) {
  $keyval = explode ('=', $keyval);
  if (count($keyval) == 2)
    $myPost[$keyval[0]] = urldecode($keyval[1]);
}

This seems pretty risky to change, imo. Especially in the 7.x branch where we don't even offer support for Adaptive Payments. (Does PayPal itself support that API any more?) Closing this out until there's a concrete use case and some form of testability.