Fix installation failure on some systemd distros.

Likely for backward compatability reasons, /etc/init.d exists in at least some
modern systemd distributions resulting in the installer failing attempting to
run update-rc.d, which does not exist. (CentOS 7, Fedora 20)

Instead, flip the logic so we look for systemd first, and fallback on init.d.
Fix minor issue with stopping existing systemd service during install.

Syntax for systemctl list-unit-files appears to no longer be correct,
specifying a service causes an error: "Invalid number of arguments."
Merge pull request #110 from dgoodwin/install-systemd

A couple small fixes for installation on systemd distros.