* origin/pr/267:
fix for ArchLinux: notify dom0 about installed updates The launch of the qubes-update-check service failed on ArchLinux, because the qubes-rpc uses the `service` command which isn't available for this OS.
fix archlinux detection of available upgrades note: checkupdates return 2 when no updates are available (source: man page and source code)
upgrades-installed-check requires pacman-contrib for checkupdates
