Dashboard on /admin empty with 3.x-dev

pwolanin mentioned that the /admin dashboard is broken with 3.x-dev, since D6 has the "navigation" menu hard-coded.

Probably needs some new hook_menu_alter() functionality to swap in some replacements functions. Funky, tricky, and probably only serves for <1% of all users of admin_menu, because I can't recall when I visited that (superfluous) page on one of my sites last.

Thus, minor.