meson: build systemd using meson

      It's crucial that we can build systemd using VS2010!

... er, wait, no, that's not the official reason. We need to shed old systems
by requring python 3! Oh, no, it's something else. Maybe we need to throw out
345 years of knowlege accumulated in autotools? Whatever, this new thing is
cool and shiny, let's use it.

This is not complete, I'm throwing it out here for your amusement and critique.

- rules for sd-boot are missing. Those might be quite complicated.

- rules for tests are missing too. Those are probably quite simple and
  repetitive, but there's lots of them.

- it's likely that I didn't get all the conditions right, I only tested "full"
  compilation where most deps are provided and nothing is disabled.

- busname.target and all .busname units are skipped on purpose.

  Otherwise, installation into $DESTDIR has the same list of files and the
  autoconf install, except for .la files.

It'd be great if people had a careful look at all the libr