This service monitors (each 3s) switchdev ports and brings down CPU
ports when all related non-CPU vlan ports are also down. Otherwise,
it brings the port up.

In order to hide CPU ports from netifd, when a device is brought down,
the device is renamed adding the suffix "_down".

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>