General improvements for machtype

[jdreed]

There are a couple of failure modes with machtype:

• The first is that it's possible to screw up an entire release by using the wrong source version of machtype. It's a bit better with the Python rewrite, because it will now FTBFS if it looks like something is wrong.
• machtype is treated like a native package (due to legacy issues with autogoo), but it's totally not, because it hardcodes things like dpkg-query. We could _make_ it a native package by keeping it linux-specific, and then supporting a machtype.d directory with query snippets (c.f. athinfo.defs.d) for distro-specific stuff. This might slow it down a bit, but I don't think it would be appreciably slower. But we should either do this, or move it to the debathena/ part of the repository.