Make it easier to find porterboxes and buildds

This series provides some affordances for machines.cgi to make it easier to find porterboxes and buildds in the list.

There are also a few pre-patches to make testing this easier.

My testing has been ad-hoc: I have not installed this as an actual CGI. But I have done various things that I think show that my new stuff works and I doubt it will break it. If it breaks when deployed I promise to fix it ASAP, if I've had a bit of warning so I'm actually about...