New site creation broken against trunk

Core "deprecated" `wpmu_current_site()` in https://core.trac.wordpress.org/changeset/27359. In trunk right now, it's:

```
/**
 * @todo deprecate
 */
function wpmu_current_site() {
}
```

This means that our use of it in `wp site create` without a `--network-id` results in a broken site.

Conversation: http://irclogs.wordpress.org/chanlog.php?channel=wordpress-dev&day=2014-03-04&sort=asc#m803826