Clean up `bp_has_message_threads()`:

* Move default box ahead of argument parsing so it can be overridden VS being forced.
* Remove unused `$bp` global.
* Use `bp_parse_args()` so arguments can be filtered before they are queried.
* Pass `$r` into `bp_has_message_threads` filter.
* Add inline documentation and phpdoc block.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7924 cdf35c40-ae34-48e0-9cc9-0c9da1808c22