Add separate udev rules for systemd installs, rework service

On systemd-based systemd, running bootmac directly from the udev rules doesn't work very well. As previously noted, using a systemd service instead fixes those problems, although it requires finding an appropriate "reverse dependency" (such as e.g. bluetooth.target).

Using unit activation from udev makes it easier, at it will be triggered once the interface exists. Moreover, in order to avoid creating multiple service files, we can make it a template and use the instance parameter as bootmac's main command-line argument.