All other rules in 99-systemd.rules are pretty much concerned about
SYSTEMD_READY=1 and tagging devices the service manager shall track. But
the gpt-root symlink is different, and it makes sense independently of
the service manager. Hence give it a separate file.
This introduces indentation for inner sections skipped via GOTO=. The
rules parser supports that. In order to make this more readable, let's
maybe make use of this.
