Various notes on /usr unification

And the Fedora ARM dept has realised that they are going to need a lot of ARM build machinery if they wish to continue doing that (because it still takes a whole heap longer - although of course this improves over time).

Mass rebuilds are good for checking everything is still buildable (Debian regularly finds old stuff that doesn't actually rebuild anymore due to changes around it, because we do in fact do test mass rebuilds on x86 from trime to time), and it propogates toolchain improvements. But it does need much more build resource than incremental building. The more arches you support the harder it is to do.