Default to the component directory root_slug in BP_Component::setup_globals()

This change will make BP_Component extensions a bit easier to build, as it will
no longer be necessary to fish the component root_slug out of bp_pages
manually (in the majority of standard cases).

Fixes #3578

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7465 cdf35c40-ae34-48e0-9cc9-0c9da1808c22