Avoid a conflict between member's profile url and member's blog url

When root profiles are enabled on a subdirectory multisite config, we need to make sure the member's profile url is different than the member's blog url. To do so the username needs to be an illegal blog name on this particular config.

fixes #5332

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8684 cdf35c40-ae34-48e0-9cc9-0c9da1808c22