Archlinux support

This adds Archlinux support. The systemd methods should be reusable with Fedora, RHEL/CentOS 7 and other systemd implementations without much modification, but for now it is only for Arch.

Currently it has 3 deficiencies compared to the RHEL/Debian implementations:
1. It ignores the package version (and prints a warning that it will be ignored) since only the latest version is supported on Arch, and their repo format only allows one version of a given package name there will never be multiple versions in the repo at once. This should be a non-issue.
2. It ignores the use_upstream_package_source variable as there is no other upstream for the Arch packages. This should be a non-issue.
3. It does not utilise the 'use_name' param for run.pp, it always uses this mode as I could not get writing/reading the CID to work correctly inside of a systemd service unit. I'm not sure how you would feel about that since it is a slight limitation, but feel free to fix it if you can figure it out.

Other than that all functions that work on the other supported OSes appear to be working fine from my (albeit limited) testing.