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.

bootmac-bluetooth.service

-[Unit]
-Description=Set Bluetooth Mac Address
-PartOf=bluetooth.target
-
-[Service]
-Type=oneshot
-RemainAfterExit=yes
-ExecStart=/usr/bin/bootmac --bluetooth
-
-[Install]
-WantedBy=bluetooth.target

systemd/bootmac-bluetooth.rules

+#
+# Configure the MAC addresses of the Bluetooth interface when it appears.
+# Copyright (c) Dylan Van Assche (2022)
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+
+ACTION=="add", SUBSYSTEM=="bluetooth", KERNEL=="hci0", TAG+="systemd", ENV{SYSTEMD_WANTS}+="bootmac@bluetooth.service"

systemd/bootmac@.service

+[Unit]
+Description=Set %i MAC address
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/bin/bootmac --%i