Export issues on multisite configurations

[yoavf]

In 3.0, some changes were introduced to the export functions that may cause the export to fail some multi-sites configuration.

For example, [14153]

This new query won't work on installations where the posts table and the users tables aren't on the same server.

2nd issue:
from wp-admin/includes/export.php

$where .= $wpdb->prepare( "AND ID IN (SELECT object_id FROM $wpdb->term_relationships WHERE term_taxonomy_id = %d) ", $term_id );

This is flawed because it assumes the term_relationship object_id is the same as the term_id which might not always be the case (ie global tags configuration).

The attached patch fixes that by using get_objects_in_term() instead. The authors selection drop down code is reverted to pre-3.0 code.

[nacin]

Perhaps we can use cache_users() to prevent so many get_userdata() calls. Ordering by display_name perhaps using array functions would be a nice UX improvement too.

[nacin]

First issue is no longer an issue (for now).

Assigning the other to duck_.

[duck_]

We're okay on both (for now), since both are issues with filtering.

I had noticed the weirdness in selecting tt_id as t_id the other day when quickly looking at what might have to happen to re-implement importing but hadn't spotted this ticket...

Will need to remember this when reintroducing filtering, #15197.