(#20421) Use systemd as default service on arch

Per https://www.archlinux.org/news/end-of-initscripts-support/ Arch
Linux has migrated to systemd and as of January 2013 has dropped support
for sysv style init scripts. GH-1468 updated the systemctl binary
location which made the systemd service provider functional on Arch, but
since there was no clear default between init and systemd, init won out
which effectively broke systemd init scripts.

This commit sets systemd as the default for Arch Linux, and makes the
sysvinit scripts available as a fallback option. Since the init provider
is still functional and can be delegated to if there is no systemd
service for a given system service.

Squashed into a single commit and commit message update by Adrien Thebo
<adrien@puppetlabs.com>
Merge branch 'pull-1615' into stable
Merge branch 'stable'