And also use systemd-tmpfiles for that directory creation. Fixes QubesOS/qubes-issues#1401
There were multiple problems with reusing existing one: - need to sync with upstream changes (configuration path etc) - conflicts resolution on updates - lack of iptables --wait, which causes firewall fail to load sometimes QubesOS/qubes-issues#1067
It is no longer Fedora-only proxy, so rename to not confuse the user. Also documentation refer to it as "updates proxy" for a long time.
