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 for bluetooth. 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 service files for both wlan and bluetooth, we can make it a
template and use the instance parameter as bootmac's main command-line
argument.