Prevent BP_User_Query from fetching spam users on alphabetical sort

BP_User_Query's 'alphabetical' sort type, in contrast to the other values of
'type', does not look at users' last_activity usermeta value. As a result, it
cannot infer spam/deleted/activated status in the same way as the other 'type'
values. To prevent spammed/deleted/unactivated users from being returned on
alphabetical sorts, we do a subquery that limits results to user_status = 0.

Fixes #5114

Props sbrajesh, imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7638 cdf35c40-ae34-48e0-9cc9-0c9da1808c22