Make sure the register item of a dynamic BuddyPress nav menu is displayed to a logged out user

In case the slug of the Register directory page has been changed, the register item was not displayed to logged out user.
Instead of the Register directory page slug, we need to use the string "register" in a similar approach to the login item.

Fixes #5322

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7734 cdf35c40-ae34-48e0-9cc9-0c9da1808c22