Using www-data was a temporary measure while I was waiting for the
official nginx.org packages to be released for Debian 9 and we had
to use Debian's own nginx package.
Sync with latest packaged version from Debian 9. Effectively this
only updates comments and disabled options, but having less changes
to look at during an apt upgrade is nice and reduces the chance of
adding errors.
Debian 9 is still in beta and doesn't have nginx.org builds yet, so
we need to use the nginx package in Debian's repositories, and that
required a bit of a different configuration.
After official nginx.org builds are released we can revert this.
