systemd support

[evgeni]

Arch ships a service file, that starts the service as a real service, with dependencies etc [1].
Our version, which came from Fedora (and is used in the AUR version of the package [2]), relies on activation by udev. This is IMHO cool (esp as it enables hotplug for new disks), but is confusing for users, as they cannot control hdapsd via systemctl.

[1] https://projects.archlinux.org/svntogit/community.git/tree/trunk/hdapsd.systemd?h=packages/hdapsd
[2] https://aur.archlinux.org/packages/hdapsd-git

[evgeni]

added non-udev unit file, which is not installed by default. forgot to ref the issue in the commits (11c97e3, 0d085bf)