Automatic profile selection based on host availability

This introduces support for host availability testing via [arptest](https://github.com/Xenopathic/arptest). The syntax is simple; the following will test for either a host at 192.168.2.1 or a host with MAC address aa:bb:cc:dd:ee:ff at 192.168.2.2:

    Tests=('192.168.2.1' '192.168.2.2|aa:bb:cc:dd:ee:ff')

This is implemented as part of the ifplugd action.

Only issue with integrating this into Arch is packaging - I have packaged arptest into the AUR at https://aur.archlinux.org/packages/arptest-git/, however netctl is in core, so it would likewise need to be packaged there or in one of the other official repos.

No IPv6 support yet, although it is just a matter of porting arptest to the IPv6 protocols (Neighbor Discovery Protocol).