Add header_register_callback(), allows a userland function

Add header_register_callback(), allows a userland function
to be called as all the headers are being sent and after all
of the default headers have been merged.

headers_list(), header_remove() and header() can all be used
inside the callback.

<?php

header('Content-Type: text/plain');
header('X-Test: foo');

function foo() {
  foreach (headers_list() as $header) {
    if (strpos($header, 'X-Powered') !== false) {
      header_remove('X-Powered-By');
    }
    header_remove('X-Test');
  }
}

$result = header_register_callback('foo');
echo "a";