systemd support for Arch Linux guests

Arch has recently changed to using systemd for the initialization process, and thus making /etc/rc.conf obsolete for their boot process.

The networking config support in vagrant depended on rc.conf being present, and also on using "netcfg". I changed it to support their new style, in accordance with the updated installation guides.

I also changed the hostname setup and shutdown to their recommendations.

The base box I was trying to create now works without any issues with this modifications.

Note these changes are based on 1-0-stable, as I wanted a quick fix for the issues I was having with 1.0.5, but I think they could be brought to master too.