rfkill: use ID_PATH as identifier for rfkill state files

Let's include the stable device path for the rfkill devices in the name
of the file we store the rfkill state in, so that we have some stability
regarding enumeration order.
backlight: include ID_PATH in file names for backlight settings

Much like for rfkill devices we should provide some stability regarding
enumeration order, hence include the stable bits of the device path in
the file name we store settings under.
git: add test-ellipsize to files to ignore by git