Drop reference to /run/initramfs/upgrade.conf

For some reason upgrade-switch-root.target was looking for upgrade.conf.
This is silly, since we don't create it anymore.
Fix --network VERSION if /etc/debian_release exists (#1057817)

It turns out that platform.linux_distribution() will return the Debian
release version string rather than the Fedora one if /etc/debian_release
exists.

Since fedup is designed for Fedora (and its derivatives), only check the
release files for Fedora (and its derivatives).