debian/changelog: housekeeping

debian/changelog: housekeeping

I was rushing a bit two days ago, and started a new version that's not
going to end up in Debian unstable and it's also not tagged, so let's
continue development.

* Cleanup all entries to not include versions that were never actually
  released.
* Add more summary information about the Spectre/Meltdown related
  upstream changes.
* Use an extra leading ~ during development, so that intermediate test
  builds never result in files with the same names as the actual build
  that will be released later.