Fix regression in `bbPress1.php` importer introduced in r5349 where topics were imported a second time as part of the reply section import.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5533 9866e705-20ec-0310-96e7-cbb4277adcfb
Better ABSPATH check in `bbpress.php` stub loader.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5534 9866e705-20ec-0310-96e7-cbb4277adcfb
Vanilla2: Don't import Vanilla 2's deleted users

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5535 9866e705-20ec-0310-96e7-cbb4277adcfb
Drupal7: Don't import Drupal 7's guest user

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5536 9866e705-20ec-0310-96e7-cbb4277adcfb
Vanilla2 importer improvements: 
* Don't import Vanilla 2's deleted topic or replies as both topic and reply content is deleted resulting in fragmented topic context
* Use `callback_topicid_to_forumid` to convert replies parent topic id to forum id eliminating a SQL join improving performance.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5537 9866e705-20ec-0310-96e7-cbb4277adcfb
Introduce support for importing anonymous topics and replies in `converter.php`
* Adds two new steps to the import conversion process:
 * "Convert anonymous topic authors" and "Convert anonymous reply authors"
* Utilizes existing bbPress support for anonymous user topics and replies
 * Adds post meta key `_bbp_anonymous_name` to imported anonymous topic and replies
 * Depending on the forum being imported these may be referenced as anonymous, guest or deleted topics or replies
* Imported topics and replies are assigned post meta key `_bbp_old_topic_author_name_id` or `_bbp_old_reply_author_name_id` respectively that is then compared using the returned `true` or `false` in the callback function `callback_check_anonymous()` of each of importers that are are supported.
Props netweb. See #2347

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5538 9866e705-20ec-0310-96e7-cbb4277adcfb
Include anonymous topic and reply import support in `phpBB.php` and `SMF.php` importers
Props netweb. See #2347

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5539 9866e705-20ec-0310-96e7-cbb4277adcfb
Merge branch 'master' of git://bbpress.git.wordpress.org