Update to 6.7.0

A very messy first stab. I'm doing this mostly to see whether there are any bugs introduced by the Meson rewrite that can be fixed before 6.7.0 is out. So far I've found at least one :)

In the process I've also spotted a few issues and areas of improvements for downstream, which are independent of the Meson rewrite and so I've sent separately as !59 (merged), !60 (merged) and !61 (merged). Hopefully we can merge those quickly, along with !58 (merged), and upload 6.6.0-1 in a few days; for the time being, those commits are also included here.

I've gotten to the point where I can build the package but not much further. The list of files that end up in the binary packages looks fairly similar, so that's promising, but I haven't looked at their contents yet.

Lintian reports a bunch of systemd-service-file-refers-to-var-run, which are probably easy to fix, and a bunch of much scarier custom-library-search-path. I'll try to figure out whether that's something that requires upstream work.