homework: teach home_lock() + home_unlock() + home_deactivate() to use HomeSetup...

homework: teach home_lock() + home_unlock() + home_deactivate() to use HomeSetup, too

This is just some minor refactoring, to make these two operations work
like the rest.

home_lock_luks() will now use the root_fd field of HomeSetup already,
but for home_unlock_luks() + home_deactivate() this change has no effect for now. (But a
later commit will change this.)