Add and verify working, canonical rel prev/next link tags

#5059 

#### Changes

* Added `template_preprocess_HOOK()` for `theme_pager_link()` to both os_basetheme and hwpi_basetheme.
* In both hooks, added code like below which adds a `<link />` tag to the head of the HTML document for the first occurance of each of: previous and next pager links.

#### New code:

```php
<?php

// Begin customizations not in drupal core pager.inc
// Pagination with rel=“next” and rel=“prev”. Does not support well multiple
// pagers on the same page - it will create relnext and relprev links
// in header for that case only for the first pager that is rendered.
// @see www/includes/pager.inc
static $rel_prev = FALSE, $rel_next = FALSE;
if (!$rel_prev && $text == t('‹ previous')) {
  $rel_prev = TRUE;
  drupal_add_html_head_link(array('rel' => 'prev', 'href' => url($_GET['q'], array('query' => array_filter($query, '_hwpi_basetheme_not_empty')))));
}
if (!$rel_next && $text == t('next ›')) {
  $rel_next = TRUE;
  drupal_add_html_head_link(array('rel' => 'next', 'href' => url($_GET['q'], array('query' => array_filter($query, '_hwpi_basetheme_not_empty')))));
}
// End customizations not in drupal core pager.inc
```